I spent a week with them, at first listen I was wondering why anyone in their right mind would like them, was very cold and sterile sounding to my ears (almost too accurate), but yet I was drawn to them for some reason, by day three I was loving what I heard. The sound absolutely comes from the front and back of you rather then the sides . The bass is accurate, tight and controlled, nothing I own comes close to them in that respect. The sound stage was massive. The highs were a bit cold and sterile to my ears, but I just came off an Lcd-2 kick for like two months.
I did not have that moment of chills when a good vocal comes in the way the 650/Lcd-2 hits me every so often, but I definitely liked them a bunch and almost pulled off a major trade(lots of my collection) for them. I am glad he changed his mind, once I got back the loaned Lcd-2 it was like seeing an old friend, I missed the warmth and intimacy of the vocals and the(maybe a tad inaccurate) bass and mids.
No other head phone I have ever owned/heard has the precision of presentation that I heard while auditioning the HD800, but like I said I did not have that "moment where the hairs stand up" that I get from some of my other cans.
This is basically why I haven't gone after the hd800. The most important part of the signature is the mids. Vocals are the things that really do it for me, followed by deep extended bass with good texture. Treble is by far the least important aspect for me. I like treble that is a touch darker. With all that said, the 650 is right in my wheel house.