Axe-Fx II or AX8 for effects only
From Axe-Fx II Wiki
As effects-only device in an amp's effect loop
- When using the Axe-Fx II or AX8 as an external effects-only device in the effects loop of an amp, you probably want it to send and receive LINE level signals. So use the input(s) at the rear, not Instrument. When using Input 1 (rear) on the Axe-Fx II, set Input 1 Select to Rear in I/O.
- Adjust I/O > Input Level for an optimal signal-to-noise ratio.
- Select the correct output mode in I/O.
- Use the physical output knob to get the required output level.
- Cliff: "You should NOT use Boost/Pad in this configuration." source
As effects-only device before an amp
- When placing the device in front of an amp:
- Set Boost/Pad to 12 dB (to make sure the full range of the D/A converter is used).
- Turn up Output 1 Level all the way.
Creating presets for effects-only
- To make a signal go through the Axe-Fx II or AX8, connect the grid input to output using shunts.
- Replace the shunts with the effect blocks you need.
- Do not add Amp blocks or Cabinet blocks. The TUBE PRE amp type may be an exception to warm up tones. And in some cases an user cab can be useful too, as a container for special IRs (i.e. Echoplex preamp coloring).