What do you mean, that sounds like it would be delicious.
Actually, I'm more impressed by the O2 than the ODAC. The ODAC sounds different depending upon what it's plugged into. The O2s main issues with me are treble stridency and some weakness in the bass. Both can be remedied with an op-amp swap to the LM4592 which I have done myself. The O2 excels is how clean and controlled it sounds. However, the O2 does sound flat in terms of both micro and macro dynamics, and does not extract detail anywhere as well as better amps, e.g UHA-6S, Schiit Mjolnir, etc. Heck, even a slightly colored tube hybrid amp such as the Melos SHA-1 is does dynamics and resolution better. This is of course dependent upon source and transducer quality: DAC, recording, and headphones.
Of course YMMV. The fact that I would be hard pressed to find anything better for less than ~$279 speaks volumes about the O2.
How many amps and dacs have you heard?
I'm using an O2 with HD 650s. How much gain are you guys using for normal listening? I usually have mine about 2 bars. If I want to really hear everything in the mix I set it up to a little less than half way.
I think that low gain (2.5X) should be just fine for the 650s. They're really not that hard to drive.
Hm, how much gain? Clearly that depends on the output level of the source you're amplifying, the headphones too.
None of my headphones need incredible amounts of voltage, so with a 1V rms source I just have 1x and 3.1x gains. It's on 1x except for some classical music recordings with the relatively insensitive headphones. With HD 650s I'd be using 1x for myself, most of the time.
Just a couple heads-up:
I think purrin's got a typo there. You probably want an LM4562 (not LM4592, which doesn't exist? or at least isn't common). Hopefully this saves somebody some grief in looking up the part number.
Secondly, there are some uncharacteristically brief THD and noise measurements of LM4562 in O2 circuit at that place (August), just for reference, to compare to whatever you hear.
From my limited knowledge of amps, it will have tighter bass that extends better but it won't get heavier unless you EQ it.