rapppakko21I don't have the HD6XX but, I do have the HD650's. On my gear, the HD650's (Mimby/Jotenheim/Little Dot 1+) are superior. These however are really really nice. They are a bit bright and could use a tad more mid bass but, their sound stage is amazing. I'm definitely keeping mine. I need to spend more time with them as I only received them today. I'm currently listening to Markus Schulz Amsterdam 08 with them using a MIcca Origen+.
rapppakko21K7XX requires more current to drive, has more treble (sounds more airy/analytical), and has a larger soundstage. There's a dip around 3-5kHz that makes cymbals/wooden instruments sound slightly more hollow. The HD6XX sounds more "intimate" and "easy to listen to" due to the smaller soundstage and less treble. The differences however are pretty minor and I'm not really sure which I like better.