Have just ordered the HD650 and a Russ Andrews HC-2 cable. My current Q701s are musical and clear, but all the burn-in that I throw at them can't get rid of uncontrolled digital glare on upper orchestral strings in some recordings. Also, the head-band is uncomfortable on my bald head. Audiolab M-DAC + Lovely Cube btw.
I find my HD650s direct from the M-DAC headphone out (source: Mac Mini/Audirvana) deliver excellent detail and positioning. Timbral accuracy?... the jury is still out on that one.
Look forward to reading your impressions.