I've owned all three now and I had the 600 and 650 for a while side by side. I think it has a lot to do with system synergy and personal listening preferences. To my ears they both sound very good, especially when paired with a complementary after-market cable. For me, with the PPA, the somewhat drier top end of the 600s coupled with leaner bass didn't pair as well as the 650s so I sold the 600s to Ian (en480c4 here) who loves them with his EC lunchbox. Since then I've picked up a pair of 580s and modded them with 600 grills. Without an amp, when I'm watching movies or TV through my Senn DSP, I find the 580s easier to listen to than the 650s.