If you don't already have one I'd suggest you think about getting a good amp for the 650's as opposed to buying the 700 without one. Might be a far greater improvement than just getting new phones. I got a tubed amp for my 650's and am quite happy.
Sennheiser hd 650 vs hd700
Does anyone have a strongly held view on whether the 700 is worth 2x the 650? I will be using computer source thru a rega dac and rega ear. I know it is not twice as good, but given my tendency to buy things 20 per cent different. My plan b if I don't get the 700 is to get the 650 and wait on a deal for beyerdynamic t1s. Thanks.