The O2 "doesn't do the HE-400s" justice. Can you please elaborate? This is by far the possibly best inexpensive, high power match to the HE-400 so far that I can find. Using the same calculation used in my other post, (.5 Ohm output resistance), a 50 Ohm load should get approx 960 mWs from an O2 (beating the Schiit by virtue of lower output impedance only). This assumption may or may not hold for other factors.
I am mostly interested in loudness provided by your O2. if you think the sound quality is poor, elaborate, but treat it as a separate issue.
It lacks dynamics and body to my ears at low volume and high volume. One can only give opinions on here, my opinion is I do not like it one bit with the HE-400. I love it with the D7ks and some of my other cans.