Just got my HD650 yesterday from UPS. This is a really nice set of cans! I'm surprised that I already feel this way, despite the grip of death, lack of burn-in, and the fact that I'm currently separated from my favorite amplification option.
Hard to say what contributes to this really nice impression ... I like the laid-back attitude that these headphones project. I like that they don't seem to be amp-picky. The mids are warm and inviting. Plenty of detail. I'm quite happy with the bass -- it's not overwhelming, as with other headphones.
And the imaging and instrument placement is outstanding. Wide. Not the surreal imaging of my AKG, but that is a good thing. I was startled by the placement of voices and instruments in familiar songs at first, and then realized that this is what happens with great balance and stereo imaging.
Much more listening to occur --- I'm just very pleased that after 15 years on the market, I've finally got myself a pair of HD650's. I don't buy headphones that often and every time I've added a new set of cans, I've always had to make some kind of dac or amp change to arrive at "this sounds good to me now." Pleased that these Senn's are so nicely engineered and forgiving that they are a pleasant discovery right out of the box. Looking forward to continued tweaking and improvements!