Hey I'm in with you on that. Every time I've tried the HD650, whether it be SS amps, Schiit Amps, Woo Audio amps, Bottlehead Crack amp, I have not liked its sound. To me it remains a mystery as to why so many people like it. For the ~$300 street price nowadays it sounds pretty good though, although I prefer the sound signature of an AKG K 701.
That being said, the O2 drives the K 701 quite well from my experience. It definitely sounds more detailed and the soundstage is to its maximum potential with it versus Woo Audio and Schiit products. When I brought the O2/ODAC to the last local Head-Fi meet I went to, people were mighty impressed with how the O2 sounded. People said it could drive an LCD-2, but not to its full potential in the bass department.