It all comes down to which combination of colorations-- or lack thereof, you like at the end of the day. It's a purely subjective hobby for the most part afterall. One of the most erratic and subjective reviewers-- Mike, over at heafonia, generally agrees that the O2 is a solid amp, but hates it on most of the popular cans on head-fi like HD650, he says (the amp-- not the headphones) sounds too thin and anemic with the HD650. With the HE-400 he loves the O2. Wonder why that is?
Hating the O2 with HD650 is one of the rare times I agree with Mike. I can't stand it. I wouldn't call it anemic per-se, but it's studio-flat-analytical sounding in the worst sort of way. Very unlistenable to me, it's like listening to the bits off the disc instead of music. Impressive that HD650 can be so analytical and revealing...it demonstrates why it was so popular for so long for classical engineering studio work
HE-400 is definitely much more tolerable on O2, and I can even see why some people could like it. It's easily listenable but still makes HE-400 tilt toward the sharp detil and analytics rather than the more euphonic sounding Lyr (or many other amps, both tube and SS.) For those who like it etched and analytical it's a good match. Persionally I prefer O2 with headphones that are kind of out of control or tend to distort more euphonically on their own like Denons. For more flat headphones I always prefer a warmer, more euphonic amp
As you say, it's all preference.