I think Tyll nails it in this comparison ....
Basically for me the hd6** headphones sound more alike than not. The main differences are hd650 has slight more tilted balance toward bass so at similar volumes the hd650 sounds ever so slightly darker and possibly as a result a little 'cleaner' sounding . I actually put this down to the lack of 'air' From the more balanced hd600. I don't think either has better anything, the quality of the sound is identical and neither gives away anything to the other , in isolation for example I would say the bass is the same and the treble is the same but when music is playing and all the frequencies are present the hd600 sounds more natural and neutral and like all the frequencies have equal volume . On the other hand the 650 has slightly less treble and upper mids especially around 4-6 kHz .this gives them a very slightly darker signature which makes them better for people that prefer a slight more focus on bass and lower mids rather than treble. It's a very small difference and it comes down to your preference.Both are amazing headphones .
Thanks for your comparison and link.
From what you describe, which is very much in line with other comments I've read elsewhere, the 650's have more appeal to me due to slighter more bass and lower mids
Nice 1 - thanks again