The only thing I can ascertain from the specs is that it appears as though Flux Lab amps have much higher gain settings than the two Schiits; all of their amps appear to have over 10db of gain on their minimum level, and up to 20+ on the highest levels, meaning highest gain is what, audibly 4 times as loud as the source? Are you using the FA-12's balanced output or SE? Are you running in an exclusive mode with your DAC? Seems like the difference between the two Schiits could merely be of topology, not necessarily of one being rather that much powerful than the other. Could also be that the extra power of the Asgard means being able to drive heads with much higher impedance, not necessarily driving the same, fairly low impedance (though low sensitivity as well) headphones noticeably louder, though someone correct me if I'm way off base with that one.
How do the pot positions fair for what you'd deem a comfortable listening level, out of curiosity? To me, if they're the same give a few degrees at your preferred listening level, then for the purposes of your fostexs and your preferred listening level, they're about the same power. Plus all those power numbers from manufacturers can be a bit fudged, as we've seen from Amirm's reviews, or simply tested in incongruent ways between manufacturers.
I run fully balanced from my zen dac to my drop thx amp to my casns, but testing my HE-4XX's, about the same sensitivity, 15 ohm lower impedance though, completely single ended, full pot medium gain (1x) is pretty painful, and full pot low gain (0.33x) is pretty close to the highest I'd listen to comfortably. Not sure how the THX compares to any of the mentioned amps, or if 15 ohm is significant or not, but I hope that info is useful to some degree.
