Warning: subjective opinion follows-
I listened to a bunch of cans a few weeks back at Head-Fi's annual Can Jam that happened to be in town. I found I really liked the 650's especially for the bit of top end extension and clarity that I found missing in the 600's by comparison, and found that as many report, they needed good power to really sound right. 650's were everywhere there so I got to listen to them though lots of different rigs and really liked the HeadRoom Ultra Micro and Desktop amps with them where they sounded great. The 650's were less 'laid back' Senn sounding, but still lush, and not nearly as bright as the AKG sound (at least the 701's I listened to and also liked but seemed to have some upper midrange peaks that grated on me and made them a bit too bright for my ear). I really feel in love with the Stax sound (never listened to them before) but made the mistake of listening to the 'expensive pair' as well as the more reasonable ones.