If you don't like the "HE-400s sound signature characteristics", why own them? Seriously? Also, when you eat chicken in the evening, do you prefer Red Wine or White?
Ummmm, isn't the point that the Asgard one was already producing the volume he wanted the point with the EQ settings he used? Even if he sacrificed half the power going to an O2, he is only giving up another 3 dB or so, hardly a HUGE deal . . . . If he had a good amp already, I wouldn't have bothered "promoting" the O2.
Any headphone with low impedance could be described as requiring "more current" . . . how is that helpful in choosing an amp? Hint: Its not. All you need to know is the power output at the rated impedance of the cans, its not rocket science. Claiming "planars require more current" is totally unqualified information and a bit misleading to say the least.
Seriously, I think it is time to line up 10 speakers with 10 amps and play with the combinations of impedance, power, and SPLs until the relationship between the factors sink in. Let me know if you can actually hear one thing during your experiment that has anything at all to do with the amp independent of the speakers themselves (maybe a fuse will pop when you wire down to 1 Ohm mono and play a test tone for an hour). In my car, I wired 3 subwoofers (2 identical, 1 of different brand) in series (~12 Ohms mono) and as a result fed the trio maybe 60 watts total. F'in heresy according to all audio wisdom (low power, messed up impedance, etc). If I drop to 4 Ohms mono, I get close to 200 watts, but lose the SPL produced by having 2 other drivers as well as their unique "sound signature". Then I ran two drivers. Where was the "strained bass"? Where was the loss of "dynamics"? Where was the increased noise and distortion? I'll give you a hint - it never came. The only difference was that I could run a single driver close to its limit (hence risking clipping and distortion that might be audible), while I couldn't with less power. Suspension travel (i.e. a mechanical limit) surfaced on the 8 inch driver, while the 10 inch driver was willing to be pushed more. That is about it.
Not only is the amp completely unemotional about my choices, but the maximum SPL is approximately the same given the drivers I chose. I get a slight tactile "enhancement" from running the larger drivers, but that is it. There is a very, boring, very utilitarian function that amps must provide. The manufactures labor tirelessly to make you believe that they are not mere "commodities". But guess what . . .