I should get a chance to audition the HD800 this week. the thing about amps and dacs I mean a dac's job is to turn 1s and 0s into a wave form, and the odac does that as good as any other dac can if not better. So if you hear a difference in a 2 thousand dollar dac, that means that dac is messing the 1s and 0s and changing the sound, which is opposite of hifi, so it may or may not sound better, but technically speaking it is not doing a good job on what a dac is supposed to do.
Same with amps - an amps job is to deliver power to drive the headphones - an o2 at 6x gain has more than enough power to drive virtually any pair of headphones with minimum THD, so if a 2 thousand dollar amp sounds different, then it's messing with the signal to give an illusion of improvemen (which is not worth 2 thousand dollars imo)
That being said I do enjoy my OTL tube amp with vintage mullard tubes with my 650hd because I like how it Physically changed the sound to be more soft and pleasurable - but if i spend 1000-1600$ on headphones, i don't like the idea of also having to spend 3000$ on a dac/amp just to fix the headphones, because if it doesn't sound good on the o2odac, then thats mostly on the headphones. But i am hoping the hd800 will be enjoyable with my tubes.
This exactly was my attitude before when I got into this hobby. I've read NWAVGUY's website like my Bible, it just made sense, as everything was backed by numbers and figures. Then, at my first head-fi meet, I have heard the same hd800 hooked up on a liquid gold. Since then, my life has changed, and I'm not exaggerating here. I know you are trying to justify why you shouldn't spend more dough on high end amps and dacs but do yourself a favour and get to a meet to listen to it with your own ears!