I didn't like the Senn 600 or 650. Both of them sounded dark and muddy with the headphone amp I had (Headroom MicroAmp). They also squeezed my TMJ area and were quite uncomfortable. Sold them both, got AKG K702. Tight bass, more midrange detail, smooth highs, not fatiguing to wear (but some people dislike the bumps on the underside of the headband).
I like them even better with the Burson HA-160 amp I got recently.
AT&T U-Verse Northeast Ohio
Denon x4000, Carver m1.0t, Samsung LED TV, B&W 704 mains, two M&K subwoofers, Oppo 103, Roku 2, etc.
Headphone system: Sennheiser HD600, AKG K701, Burson HA-160, Marantz CD5004, Denon DVD-3910