I Love my HD650sI have had my HD650s for over 2 years now and they really blow me away. They run a Stefan Audio Art balanced cable to a Headroom Balanced Amp and Stello DA100S Dac.
I recently defoamed them (foam removed from the ear pads and the small foam pad on the grille side). I added nylon stocking to the ear pads and used the ear pad foam inside the grilles. The result is super tight bass, but a loss of the Senn sound. The mids also were more recessed. I EQed them and I have not looked back, plenty of detail, bass that punches hard and with the EQ, great midrange so vocals sound good... sort of a refined RS1 Grado is the best way to describe them.