Well I honestly didn't realize this thread was still going on - didn't get any notifications -, but maybe it's a good idea to make up a list of things that affect our perception of sound between AX8 and brain:
1. EQ in AX8
2. Impulse Response in AX8
3. Audio gear after AX8 (mixer, IEM transmitter/receiver, etc)
4. Drivers, quantity of them, configuration, geometry, etc.
5. On/off axis response (FRFR only)
6. Room/stage early reflections (FRFR only)
7. 2.8 kHz ear canal dip
8. 4kHz hearing dip (comes with age)
9. Fletcher-Munson curve
10. Band mix (vs guitar by itself)
Did I forget anything?
Now, my AX8 already sounds killer through FRFR, sounds very much like my tube amp right next to it through a Yamaha DXR10, no issues there.
My goal is really to minimize the difference between FRFR and IEM, that's the reason I'm looking at IRs and also a little at EQ. I'd like to reach a compromise where both sound acceptable, but I believe my standards for "acceptable" aren't exaclty the lowest.
Drivers and audio gear matter, that's why I've got a Sennheiser EW300 IEM G3 and a pair of Etymotics ER4SR, and honestly I feel I'm getting very close, just wanted to hear more ideas on the specifics in my OP. Thanks to everyone who replied.