Holy crap, I got my 650's from AA in under 24 hours from when I ordered! And I didn't get charged s/h!
Even though my headphone experience has been limited to hd280's for a year and then the 600's for 2 weeks, I can tell the difference between the 600's and the 650's. The harsh treble is gone, slightly more pronounced bass (not sure if I like that or not, but this is straight out of the box, so I expect everything to change), and the soundstage really is noticeably different. Better different.
Also, I was having comfort problems with the 600's with the headband... the grooves of foam made my head hurt after a while for some reason. That seems to be gone with the 650's, thank god.
If I ever get a real amp and source I've a feeling my rash purchase will be justified.
Oh, on a side note, when I went to UPS to return my 600's to amazon, they took it without charging me; said it was prepaid. I had it delivered with the super saving free shipping, so maybe that's why... or they'll jam me over by taking it out of my refund. Strange, either way around.