Cab block

Manual

• 5.2 Cabinet (CAB)
• 5.2.1 User Cabs
• 16.2 Table of Cab Types
• 16.3 Loading User Cab IRs
• 10.5 IR Capture
• 13 Backing Up and Restoring

User contributions

What's an Impulse Response (IR)

• An Impulse Response (IR) is a collection of data representing sound measurements taken from a speaker cabinet or system and used by the Axe-Fx II to enable the Cabinet block to emulate a particular speaker cabinet. A test signal is played through the actual speaker, recorded, and used to generate a profile utilized by the Axe-Fx II to reproduce the measured respons. The Axe-Fx II also uses IRs for microphone simulation. 
• The terms "cab", "user cab" and "IR" are often mixed up.
• More information in Wikipedia

Nearfield and farfield IRs

• Most IRs represent the tone of a speaker that was recorded with the microphone close to the speaker ("nearfield" or "close-mic'd"). "Farfield" IRs on the contrary represent the speaker sound when there's some distance between the listener and the speaker. See also Audio topics. There are 3 farfield IRs among the stock cabs, created by Jay Mitchell ("JM").

Matching amps and cabs

• It’s a matter of personal preference which cab model (IR) you want to combine with a specific amp model. You can choose for traditional combinations. Or be creative and innovative. The differences can be huge.
• When comparing cabs, don't judge too quickly. Each time you select a cab, you may need to adjust the amp settings to dial in a tone.
• Common combinations of amps and cabs are listed here: Amp: all models.

• Legendary Tones: G12M versus G12H
• TGP: EVM 12L versus EVM 12S
• Sound On Sound: Choosing guitar-amp speakers
• Sound on Sound: Understanding and recording guitar speakers
• Guitar Player: 15 12" speakers
• Rivera: 15 speakers compared (YouTube)
• Eminence tone guide
• Fenderguru: Selecting speakers
• Guitar Player: Alnico Taste Test

Cab block position on the grid

• In the "real" analog world it makes a difference if you put effects before or after the speaker cabinet. It's different with the Axe-Fx II. 
• Javajunkie: "You can place the effects loop anywhere in the chain (just add the fx loop block). Unless you are running a stereo cab or 2 mono cabs panned hard L/R, you may want to place stereo effects after the cab. The cab is a linear time invariant effect (unless you add drive) so effects like delay and reverb will sound the same before or after it. As Cliff and others have stated on numerous occasions LTI effects can be placed before and after each other and they will sound the same. Only when placed before or after non-LTI effects (drive, amps, et. al) it really matters. The one caveat there is that some effects are mono, placing effects before and after that makes a difference."
• Cliff: "The difference in having the cabinet before or after the effects is usually subtle. It depends on how non-linear or time-variant the effect is. For effects like EQ, which are linear and time-invariant, it doesn't matter at all. For slightly time-variant effects like chorus and flanger the difference isn't very pronounced. For highly time-variant effects, like pitch shifting, the difference can be marked."
• Cliff: "The cab block is level-dependent if the Motor Drive is non-zero. So if you turn up/down the level out of the amp block you may need to compensate by doing the opposite with the Motor Drive." Source
• Keep an eye on the mono/stereo configuration. When placing the Cab block at the end of the grid, set to Hi/Ultra Res or Normal Res, the output signal will be summed to mono. And if the Cab block is set to Stereo but it is followed by a mono block (such as Drive), the resulting signal is also mono.
• Cabinet blocks in parallel rows sound louder than a single Cabinet block. Here's the explanation. Bakerman: "It depends on how you're panning. Assuming a mono signal sent to cabs: Stereo cab w/ Pan L and Pan R fully left & right will be the same output level as 2 mono cabs w/ balance L & R. If pans/balances are centered the 2 mono cabs will be 6 dB louder. Balance elsewhere would be between 0 and 6 dB louder, and balance doesn't correspond 1:1 to pan L/R for the same placement. Balances will need to be further toward -50 or 50." Source

Hi/Ultra Res, Normal Res, Stereo

• The Cab block has 3 resolution modes: Hi/Ultra Res, Normal Res, Stereo.
• Normal Res: supports IR consisting of 1024 samples. You can often use Normal Res without any negative impact on the tone, compared to Hi Res.
• Stereo: supports two Normal Res IRs. CPU usage is the same as in Normal Res mode. Stereo mode does not support Ultra Res IRs; to use Ultra Res in stereo cab modeling, you need two Cab blocks in parallel, with their Input and Balance panned to each side.
• Hi/Ultra: supports Hi Res IRs (2040 samples) as well as Ultra Res IRs (up to 8000 samples). Using Hi Res or Ultra Res IRs requires more CPU power than Normal Res / Stereo. But Ultra Res is more efficient than Hi Res, which results in about 4% LESS CPU usage with MORE resolution.
• If you don't need cabinet simulation at all, for example because you're using the Axe-Fx II for effects solely or exclusively with a power amp and speaker cabinet, switch it off in the Global menu. This will decrease overall CPU utilization considerably.
• Samples translated in milliseconds:
  • Normal Res = 1024 samples = 20 ms
  • Hi Res = 2040 samples = 40 ms
  • Ultra Res = 8000 samples (max) = 170 ms (max)

Ultra Res in detail

• Firmware 13: "Added support for UltraResTM speaker IR processing. UltraRes is a proprietary technique that enhances the spectral resolution of an IR without adding CPU burden or storage requirements. Full support of UltraRes requires the CabLab utility to convert .wav files to the required data format. NOTE: UltraRes IRs do not support size warping therefore the Spkr Size parameter is unavailable for UltraRes cabinets. In Normal Resolution mode size warping is possible."
• Cliff's comments below are taken from this thread:
  • "The problem with conventional IRs is that they are too short to capture the detail in the low frequencies. There are those that maintain 20 ms is the maximum length you need to fully replicate the speaker. This would be about 1000 samples at 48 kHz. I disagree with this as I have many IRs here that exhibit significant energy beyond 20 ms. I believe the room has some influence as the low-frequency modes of the room will impact the resulting sound. The amount of this impact depends on the room, the mics, distance, etc., etc. Or perhaps certain speakers have particularly high Qs in the low frequencies. Regardless, it is my opinion that you need IRs much longer than 20 ms to fully capture the "mic'd amp in the studio" sound. My tests show that IRs of 8000 samples are required to fully capture the low-frequency detail. Unfortunately to process an 8K IR in real-time require copious processing power... Fortunately I have developed "UltraRes" cabinet modeling. UltraRes cabinet modeling provides the frequency detail of a very long IR with little or no added processing power requirements. The following image depicts the response of UltraRes cabinet IR processing: ..." (see thread)
  • "Existing IRs will still be processed as usual. UltraRes IRs will be tagged as such which will indicate to the processor to use the new processing algorithms. Note that UltraRes IR data is not conventional IR data."
  • "The frequency resolution of an IR is the sample rate divided by the number of samples in the IR. The window function has nothing to do with frequency resolution (except for making it even less). So a 1K IR at 48 kHz sample rate has a frequency resolution of roughly 48 Hz. If a speaker has a resonance (formant) at, say 80 Hz with a Q of, say, 3.0, then 48 Hz is insufficient to capture that resonance accurately. You need a frequency resolution of several Hz to accurately recreate that resonance. I chose 80 Hz and a Q of 3 because that's what that response looks like. The Q could even be higher than that. It doesn't take much mental energy to realize that if you have a narrow formant at a low frequency then you need fine frequency resolution to reproduce that. An 80 Hz formant with a Q of 3 only spans about 25 Hz. Obviously a frequency resolution of 48 Hz is not going to be able to reproduce that. Windowing only smooths the response even more. This is basic FFT theory. The less time-domain information you have, the less frequency domain information you have and vice-versa. This is the uncertainty principle. I always window IRs with a Hann window. EDIT: I broke out my impedance measurements for that Vox cabinet and the speaker resonance is 80 Hz."
  • "Another way to look at it is to think in terms of formants. That particular speaker has a pronounced 80 Hz formant. It takes well over 100 ms for the energy of that formant to decay to the point of imperceptibility. Obviously a 20 ms IR can't reproduce an event that occurs for over 100 ms. Here is a zoom of the original non-minimum-phase IR (IOW raw time response)... (see thread). You can clearly see the 80 Hz formant. There are some room reflections but they are very small. The 80 Hz formant starts well before any reflections. It's obviously a high-Q resonance as it rings for quite a while. The higher the Q, the longer it takes to decay."
  • "Here's another example. (see thread) This is one of the new OwnHammer IRs. The IR is OwnHammer_412_MAR-CB_D-120_SS_RBN-121. These IRs are 100 ms long (4800 samples). I windowed the original IR to 4K to prove a point. The blue trace is the original IR (windowed to 4K samples). The green trace is the "typical" 20 ms IR (windowed to 1K samples). The red trace is the UltraRes version."
  • "The problem is that human perception is logarithmic and IRs are a linear process. 48 Hz resolution is way more than necessary at, say, a few kHz but not nearly enough at low frequencies. The brute force solution is to use very long IRs, 8K or more. UltraRes solves this in a novel way that uses little to no extra processing power and no additional latency."
  • "Normalization is your friend. Rectangular windows are simply truncation and are generally regarded as bad practice due to extremely high sidelobe levels. The choice of window is subjective. I actually use my own custom window that is not really a Hann window but that's proprietary information. My window preserves more frequency detail while still suppressing Gibbs phenomenon. Windowing trades off frequency resolution for sidelobe suppression. My window is optimized for the unique statistics of IRs. For a random process I tend towards Bessel-Kaiser windows. IRs have unique statistics that aren't addressed by any of the standard textbook windows."
  • "It is desired that the IR be 8K samples or more."
  • "Let me state these points:
    1. We don't record guitar amps in airplane hangers or anechoic chambers. We record them in studios.
    2. When we record a guitar amp we carefully set the amp up in the studio to get the best sound "on tape". This involves moving the amp around, placing gobos, etc. When we collected the Producer's Packs IRs we spent hours arranging the amps/speakers, mics and gobos and playing through the amp and readjusting until we were satisfied. This also included adjusting the preamps and mixing board. In one studio we found that we got the best tone raising the cabs off the floor by a couple feet, orienting them towards a particular wall and placing gobos behind (this was the engineer's standard recording arrangement).
    3. At this point our objective of the IR is to capture the sound of that amp/speaker at that position in the room, with the gobos, mics, preamps, etc., etc. The goal is not to capture the raw sound of the amp/speaker in an airplane hanger or outside using a ground-plane measurement and measurement mics. That might be someone else's goal but it is not ours. IOW our goal is to treat the cab, mics, preamps, room, etc. as a whole, as a good engineer/producer would.
    4. Subsequent analysis of the data shows that there is significant energy out to 100ms and even beyond. However there is little energy beyond 200 ms or so (as it should be in a well-designed studio). This observation was the catalyst for the UltraRes algorithm. There are other observations about the statistics of the data that I cannot disclose.
    5. Some cabinets displayed noticeable resonances at low frequencies. Others did not. The frequency of these resonances were not consistent and, not coincidentally, matched the measured resonance of the impedance sweep. It is a logical conclusion, therefore, that the resonance was NOT caused by the room but by the speaker/cabinet combination. Furthermore a plot of the group delay for the raw data showed that the delay of the resonance was too short to be a room mode. Regardless, whether the resonance is from the speaker or room or mics or preamps is irrelevant. All we care about is recreating the sound of that speaker as it would be recorded as accurately as possible.
    6. Truncating an IR destroys information by definition. We don't care where the information comes from, be it the speaker or the room or the mics or the preamps. We want all the information. If a plot of the frequency response of a truncated IR differs considerably from the non-truncated version then we have lost information and concomitant accuracy.
    7. NO ONE producing commercial IRs records them in an airplane hanger, for obvious reasons. The best ones are done in a studio using the same technique we used for the Producer's Packs: setting up the cab, adjusting the position, mics, preamps, etc. and playing through the amp/cab and readjusting until the best tone is achieved. The new OwnHammer IRs are an example of this. Many, if not all, of those IRs exhibit significant energy to 100 ms (and likely beyond but the data stops at 100 ms). Truncating them to 20 ms destroys vital information. You can argue the semantics all day long. I've compared truncated and non-truncated and the difference is clearly audible. It is especially noticeable when chugging power chords. You can hear the resonance. It goes "bonggggggg" as opposed to "thuk". Most importantly it sounds "better" IMO.
    8. UltraRes is an algorithm that markedly increases accuracy. It gives the frequency resolution of a 200ms IR without additional processing overhead and no added latency.
    9. Sometimes people can't see the forest for the trees."
  • "UltraRes is especially powerful in Tone Matching applications, particularly real-time matches and was another impetus behind the development."
  • "The myopic only see the IR as a capture of the speaker's "unadulterated" response. As I stated before I believe the future is treating IRs as capturing the entire recording chain including mics, preamps, etc. and have pushing in that direction. We have already seen the fruits of this labor in the Producer Pack and OwnHammer V2 IRs. We used mainly PP and OH IRs at Axe-Fest this weekend and the results were stellar. Andy Wood's tone was among the best guitar tones I've ever heard live and we dialed it up in 10 minutes under far less than ideal conditions. It consisted of the Two Rock amp model and the EV 12L Mix IR. When you include more than the speaker response in the IR you can have low-frequency resonances that persist for tens of milliseconds or more. Truncating an IR destroys this LF information. In many cases this LF information loss would probably not be perceptible. In other cases, from experience, it can be extremely noticeable. The bottom line is that you can always remove the information if you don't want it but you can't add back what isn't there."
  • "Let me phrase this another way. An IR can consist of the "raw" speaker response plus none, one, some or all of the following: mic, preamp, room, power amp (e.g. you want to capture the response of a tube amp driving the speaker), etc. If you only care about the raw response then a short IR is all that is required. However if you want any of the other elements as part of the IR then a longer IR may be necessary. UltraRes gives you the OPTION of processing longer IRs."
  • "Rigid thinking is great for textbooks. To push new boundaries you have to throw the books away."
• Tone Match and UltraRes:
  • Cliff: "In Realtime mode the raw internal IR length is 8K which you can dump." Source
  • Cliff: "You can export the Tone Match to CabLab and create and UltraRes IR." Source
• Cliff: "If the .wav is only 40ms long there is no sense in converting to UltraRes as you won't gain anything. Over 80 ms is desirable. The maximum length supported is 170 ms or so. Anything longer than that is truncated to 170 ms." Source
• Cliff: " To get the optimum results the length should be 170 ms or more. As the length gets shorter you'll lose information. However there may not be any information to lose. It all depends on the IR. I've seen long IRs where only the first 100 ms or so is actual information and the rest is silence. OTOH I've seen 100 ms IRs where there is obviously more information but it got truncated. You lose nothing with UltraRes except the ability to change the size of the cabinet. You gain better sound and less CPU." "You can't mix UltraRes IRs as the data is not compatible. However... we foresaw that and the UltraRes conversion process produces two files: a .ir file and a .syx file. The .ir file is the raw IR data that can be imported into CabLab for mixing purposes. So CabLab can take .wav, non-UltraRes .syx and .ir files as input to the mixer section and product UltraRes .syx files." "The .ir files are included with our cabinet packs. We will not be offering .wav files. If you have the .wav file you don't need the .ir file. A .ir file can ONLY be used with CabLab. If you use the Axe-Fx II to capture IRs it will only generate .ir and/or .syx files. No .wav files are generated. The resulting data can only be used on Fractal Audio products." Source
• Sources for free UltraRes IRs

Input Select

• Firmware 13: "Added Input Select to Cabinet block. This can be used, for example, to run two Cabinet blocks in parallel for stereo processing by setting one to Left and the other to Right."
• The regular Stereo mode doesn't support UltraRes IRs. To run UltraRes IRs in stereo, use two Cab blocksin UltraRes mode, with Input Select set to Left and Right, and their Balance set Left/Right.

Microphone models and Proximity

• Don't underestimate the impact of the mic type on the tone. E.g., adding a R121 (which is a Royer 121, captured at 6", front) will add lots of highs and lows to the tone. The 57 DYN (Shure SM57) works with almost everything. Many studios and users like to combine the Royer and SM57. Cliff: "I almost always use a mic model on the non-mixed IRs. I'll typically use a 57 on one side and an R121 on the other." Source
• The "None" and "Null" settings disable mic coloration. A mic is still involved because the IRs themselves are captured with microphones. Even when a neutral mic is used to capture, such as an Earthworks mic. When capturing IRs, the mic is most often placed very close to the speaker, so the result is a close-mic'd tone. Still, selecting "none" is a good way to prevent adding additional eq-ing to the tone.
• Firmware 3.0 added the Proximity parameter. This simulates the proximity of the recording mic to the speaker. Higher numbers translate to the mic being closer to the speaker (nearfield). Lower numbers translate to the mic being further away from the source, with the lowest number providing far-field coloration. Proximity only works when a mic model is selected, including Null.
• Firmware 11 added the Proximity Frequency parameter. This allows tuning the frequency range over which the proximity effect occurs.
• Cliff: "The mic models are actually IRs. The mic IR is convolved with the speaker IR to create a composite final IR." Source

• There's great info about mics in the document Dialing in Your Tone by Red Wirez.
• Wikipedia:
  • Shure SM57
  • Shure SM58
  • Shure Beta 87A
  • Neumann U87
  • Royer R-121
  • AKG D112
  • Neumann U67
• More information:
  • http://www.recording-microphones.co.uk/index.shtml
  • http://www.barryrudolph.com/stellar/guitar.html
  • http://www.gearslutz.com/board/so-much-gear-so-little-time/201972-mic-standards.html
  • U 67
  • SM57
  • SM58
  • MD 421
  • U 87
  • D 112
• Additional information in this Wicked Wiki thread.

Factory IRs (stock cabs)

• Cliff: "The factory IRs were hand-selected by me after auditioning thousands of OH and RW and other IRs. Some of the IRs are custom mixes of mine. My rule-of-thumb was to select as neutral sounding IRs as possible. However, what I like may be much different than what others like. Some people complain the Axe-Fx sounds too bright. Others say it's not bright enough. It's a no-win situation. This is why I've been harping on capturing IRs. It's personal preference. Producers probably spend more time perfecting mic placement than anything else when getting guitar tones to tape. An IR is the same thing, it's capturing the mic and placement. Source
• Selected Red Wirez, OwnHammer and Kalthallen IRs are included with the Axe-Fx II as stock cabs. Also included are some farfield IRs by Jay Mitchell. New: Producer Pack IRs (with "Mix" in their names).
• All stock cabs are time-aligned, which means that you can mix these in the Axe-Fx II using stereo cabs.
• Firmware 10: "Added 35 cabinet models from the first of our “producer packs”. These models are custom blended IRs using multiple mics on the cabinet. Hand-tuned by the producer, these IRs are ready-to-go with little additional EQ required for mix-ready results. The first 30 of the IRs are guitar amp cabinets and the last five are bass amp cabs. Also added several artist IRs including two from James Santiago and one from John Petrucci of Dream Theater."
• The Producer Pack IRs were captured using a mix of microphones (Shure SM57, Royer 121, Sennheiser MD421 and Shure KSM32) and a Neve 1070 preamp.

User cabs (external IRs)

• If you are looking for something different than the stock cabs, try 3rd-party IRs. The Axe-Fx II has "user cab" slots for this purpose. You can upload external IR files into these slots, using Axe-Edit or a MIDI librarian.
• The Axe-Fx II and Axe-Edit will display the name of the IRs in the user cab slots. The name is contained in the sysex data of the IR file with a maximum of 32 characters. IRs can be renamed using Axe-Edit or 3rd party utilities.
• The process of uploading IRs to the Axe-Fx is described in the manual. Also, forum member Cobbler describes the process here. You can also use Fractal-Bot.
• Firmware 10: "Increased the number of User Cabinet slots to 100 (plus one scratch-pad location). Note that the upper 50 are saved in a different area of memory and are not dumped or restored when doing a system dump/restore. A “Scratch-Pad” location has also been added. This dummy location can be used to receive cabinet data but is not saved to non-volatile memory. This allows auditioning IRs without overwriting any of the user slots."
• The Axe-Fx II XL allows for storage of up to user cabs.

Sources for external IRs

• You can get Impulse Responses (IRs) from these sources, among others:
  • OwnHammer
  • RedWirez
  • Kalthallen
  • Fractal Audio Shop. Note that the Producer Pack files have a different file format, which doesn't let you mix these IRs with known mix utilities. Solution: convert them to .wav first, using this utility.
  • Silent Underground
  •  Axe-Change
  •  Forum thread
  • Free UltraRes IRs
• Here's how to create an empty user cab (IR).
  • Use a MIDI utility such as MIDI-OX or Sysex Librarian to receive a dump of an empty slot, using Utility > Preset > Dump User Cab on the hardware.

Room ambience

• Close-mic'd IRs do not contain enough samples to recreate room ambience. That's why the Axe-Fx II provides specific room ambience parameters in the Cab block. This is a dedicated reverb effect, which also works well when using the Axe-Fx with headphones.
• Also see Audio topics.

LowCut and HiCut Freq

• The Cab block provides low-pass and high-pass controls. These make it easy to tame boomy or harsh sounds. You can also use the same parameters in the Amp block, or use a separate PEQ, GEQ or Filter block. Common settings are 80-150 Hz for high-pass (cutting bass), and 5-7 kHz for low-pass (cutting treble), but YMMV.
• Cliff: "Using Low Cut in the Cab block is akin to what you would do in the studio to carve out room for the bass player." Source

Speaker Size

• Firmware 2 introduced Cabinet Size Warping. This allows the user to change the relative size of the speaker. This parameter is accessible only when the Cab is set to Mono. Cab Size was renamed to Speaker Size in firmware 5.
• Speaker Size is disabled for UltraRes IRs.

Motor Drive

• This models the effect of high power levels on the tone of the speaker. The Motor Drive parameter controls the relative drive level and, therefore, the intensity of the effect.
• Cliff: "Motor drive isn't EQ. It models efficiency reduction due to thermal effects." Source
• Cliff: "Motor Drive will cause compression if not set to zero (as it models driver compression). Otherwise the cab block is completely linear and will not cause any compression." Source
• Cliff: "What I have found is that thermal compression is somewhat noticeable and measurable. This is modeled by the Motor Drive parameter." Source
• Changing Motor Drive from 0.00 to 0.01 and above will emit an audible "click".
• When using two UltraRes cabs in a preset, don't use Motor Drive on one of them, because this will cause a hollow sound.

Air

• The Air parameter mixes some of the signal going into the Cab block with the signal leaving the Cab block. The Air Frequency parameter lets you adjust the cutoff frequency of the mixed signal. Increase the Frequency to its maximum value for a straight mix.
• If you want to listen to just the Air'd part of the signal, set the Cab to an empty user cab, and turn up Air.

Delay

• Firmware 6 added the Delay parameter. This is a "micro" delay for stereo application.
• Release notes: "When running a stereo mode, or two cab blocks in parallel, delaying one cabinet relative to the other can achieve interesting comb filter effects. A common practice in studio recording is to use multiple mics on a speaker at different distances to intentionally introduce comb filtering."

Cab-related parameters in the Amp block

• The Amp block has a couple of parameters which are closely related to the Cab block. Check the Speaker tab (Resonance parameters, Speaker Drive) and Thunk.

IR Capture: creating your own IRs

• Get tips from the following sources:
  • Mini Manual (Cab IR Capture Guide)
  • Example clip
• IR Capture doesn't work when "Copy output 1 to output 2" is enabled.
• Cliff: "The quality of the power amp isn't really that important. As long as the response is flat. The Axe-Fx does some fancy math and removes the distortion from the measurement. Most solid-state power amps are very flat, their distortion performance is usually where they vary." Source
• Cliff: "Mixer is not so important. The IR capture algorithm in the Axe-Fx removes any distortion from the measurement so even a cheap pre is fine. The mic, however, is definitely important."Source
• Cliff: 'The IR is vastly more important. Tone Matching is a nifty feature and certainly useful but you'll get far more satisfaction by concentrating on capturing good IRs. The single most important aspect of recording guitar amps is micing the amp. Therefore the single most important aspect of using your Axe-Fx is the IR. People are too hung up on "matching" or "profiling" an amp but fail to realize that when you are doing that you are basically capturing an IR. If you capture the IR separately now you have an IR that is fully separated from the amp and therefore can be used with all models. Matching and profiling cannot mathematically separate the amp's frequency response from the cabinet frequency response. Once you do this you'll be surprised at how accurate the amp models are. I do this all the time and find Tone Matching is unnecessary now (in fact many of the amp models have had their built-in matching data removed in the latest firmware). Any differences between the model and the real amp are so minuscule as to be immaterial. A little tweak of the tone stack or EQ is usually enough to remove and differences. Besides, once you get into mixing you'll realize that you'll be applying EQ anyways so tiny differences in EQ are irrelevant. Moving the mic just a small amount drastically changes the sound. The best producers have mastered micing. You can only fix so much via EQ since EQ is essentially painting with a broad brush where mic technique is akin to using a fine-point brush. Source
• Cliff: "What an IR won't capture is any speaker distortion and speaker/guitar interaction differences. Speaker distortion is mostly irrelevant though since it is typically much less than the amp distortion. Furthermore, we simulate it anyways. Speaker/guitar interaction causes reinforcement of bass and low mids so playing through a cab at moderate to high volumes will sound slightly different than playing through the IR and listening through monitors or headphones. This becomes more prevalent at higher gains because there is more reinforcement at higher gains (this is why you can get controlled feedback easier at higher gain). The difference is so minute though that it doesn't really matter. It cracks me up that people nit-pick about minute differences when those differences are minuscule in comparison to the difference between the response of two monitors (even two of the same brand and model). You can compensate for the difference in interaction by putting EQ before the amp block and boosting the bass and low mids slightly. Or use big monitors and crank 'em." [1]
• Firmware 13: "Various enhancements to IR Capture Utility (Utility->IR CAP):
  1. The utility now supports saving IRs as either standard IRs or UltraRes IRs. The default mode is UltraRes. The desired mode can be set in the Global menu.
  2. The utility now automatically loads the captured IR into the Scratch-Pad location immediately after capture for ease of audition.
  3. The Dump function now allows dumping either the formatted cabinet data or the raw IR data via an interactive menu. Raw IR data is 8K samples and can be imported to CabLab for mixing and/or conversion to cabinet files.
  4. The utility now supports remote control via MIDI. This is intended for use with CabLab to automate the IR capture process."

Acoustic tone IRs

• To emulate acoustic instruments (acoustic guitar, violin etc.), an IR of an acoustic body can help. You'll find good ones here.
• Acoustic sounds benefit from long IRs so try to keep them HiRes / UltraRes.

Substituting an IR with a Tone Match block

• You can integrate an IR in a preset by replacing the Cab block with a Tone Match block, after having captured the tone of the Cab block. Here's how.

How to empty an user cab slot on the Axe-Fx

• To empty an user cab slot on the hardware, mute all faders (or don't load any IRs) in Cab-Lab and send it to the user cab slot. Or use Axe-Manage (Axe-Edit) to delete a slot.

IRs for Axe-Fx Standard / Ultra

• IRs for the Axe-Fx Standard/Ultra must be converted to be able to use these with the Axe-Fx II. Source 
• It's no use converting 1024-point IRs to 2040 points because they don't contain the necessary data. You need the original WAV-file to create a 2040 point IR.