Just sold my 595's having upgraded to 600's. From my stereo amp HP-out the 600s were slightly better but they didn't really blow me away, probably because they weren't being driven properly.
I've just put in my shiny new Xonar Essence STX and now the sound from the 600's is incredible! Very happy considering the 600's only cost me £100 on sale from play. I don't think I would have upgraded if they were RRP.
The law of diminishing returns definitely comes into play with audio gear. The 650's are no doubt better than the 595's, but at £250 new are they 100 quid (+ the extra cash for an amp that'll do them justice) better?