Mind to do some comparisons to 6xx from memory?
I'm currently in upgrade zone (I came about some free time) and have been looking at a few headphones that I might consider closest upgrade path (elear/elex, Atticus/aeolus) but I still use my fa011s as daily driver and have been for over 6 years despite owning he500 and thx00s because they are so light, have the closest thing to my ideal FR I've heard to this day (punchy midbass, downward tilt/Nike swoosh fr, open sounding) and I can listen to them unamped straight to my computer audio out and they sound great with everything, whether mastered good or bad. I don't hear the headphones with them, I hear the music. I still cheat on them when Im in the mood for thumping subbass or the speed and clarity of planar but 80 percent of the time, they are on my head
Sorry for the late response - its been pretty crazy for a while...
I would have loved to have been able to keep the HD6XXs as I love their sound out of the amp I've got. It's actually not all that far off the FA-011s but definitely a flatter and more reference tuning. I've never really understood the whole 'veil' thing as I've only ever had them with the Audio-GD SA-31 which is a bit of a monster which outputs 10W into 40ohms and about 1.3W into 300ohms which is plenty of juice for them. I can only assume that they are veiled when not fed properly.
Really nice bass, a little on the warmer side but not much. The Fischers are a lot more impactful - they have this amazing ability to be thundering but not boomy nor overwhelm the mids. The HD6XXs have an amazing midrange, much more intimate vocals, clearer horns, acoustic guitar strums - they really are beautiful. A little more forward than the Fischers and (this will probably change somewhat as my ears get more used to them again) not quite as dark as the Fischers. The highs are crisp, clear, not super airy but again, really nice. The Fischers can get a bit excited up there at times compared to the Sennys. Definitely a wider soundstage on the Fischers. All up, I probably preferred the HD6XXs for jazz and classical but for Drum n' Bass, EDM, rock, metal and most other genres the FA-011s are my preference. They also happen to be great for jazz and classical, just not as good as the Sennys.
I will definitely get another pair when I can afford them and I also have my eyes on the Atticus/Aeolus as their tuning seems to be right in line with my preferences. The HD6XX is also close to these, but you definitely need proper amplification - a bare minimum of an O2 or a CTH from Drop but if I get them again, I'll be aiming for an OTL tube amp this time around. I'm curious that you can drive your FA-011s straight from your PC - mine sounded really subdued and way too dark and boomy straight from a PC and even from my old Fiio X1 DAP. You must have a decent soundcard or the edition you have may just be easier to drive.
As a side note, the HE500s are supposed to be a bit of a magical match with my amp and I am dying to hear the THX00s, which version do you have? I'd love to get some purplehearts. Anyway, so far am very happy again with the FA-011s...