Also, Tyll measured his HE-500 to be less efficient than spec btw. Anyway, THD isn't just THD. The sound of distortion can and will be different at the same THD with different amps, and there's also other kinds of distortion anyway. You kinda don't get the hole picture just looking at THD. Distortion can sound good after all... and bad.
Anyway, I don't really disagree with you on the ability of the o2 to drive the HE-500's, but personally I'd get an amp with more headroom as it isn't impossible to encounter distortion that wouldn't be there if the o2 could push some more juice. Never tried the amp, actually.
Agree with the first statement but let's not visit the second or we'll end up in a taliban-like quasi religious argument (as I said, I'm not a tube guy but I respect that there are different schools of thought and after all, everyone's entitled to his personal taste/opinion). If you have the occasion, give the O2 an audition. Price/quality wise it's simply incredible. My Fiio E17/E09K combo is gathering dust since I purchased it, there's simply no comparison. Of course it's a different story compared to my Violectric dac/amp setup, but given that that one cost me about 2.5k USD there'd better be some difference. Considering the price, however, the O2 is the clear winner.