Just reporting back into my experiences with the HD650.
I've been in a slight upgradtitus mood quite recently, resulting in an O2 with a new board attached on top of it, and all those magical technojazz I still don't quite understand (but it came in a kit so I had to try it out!)
For a while I was wondering how I can pursue better performance out of my headphones. That is until I managed to switch my brain from "listening" into "enjoying" the music. Then the symptoms of upgradtitus slowly dissipated.
I'd like to think that the HD650 certainly does do wonders with the right amp, but it certainly isn't too picky about them.