To me... there is a "HUGE DIFFERENCE" between the "old version" and "new version" HD650's - the new version is much brighter, clearer, and detailed. If you like the sound of Grados and ATH's (which I think are the benchmark references), then you'll like the "new version" HD650's much better - assuming they're well matched to a source and amp (which makes all the difference). That is by far... much more important than the cables.
I've never heard the HD595's, so I don't know how they compare. But... from what others have told me... I'd likely prefer the HD595 drivers to the "old version" HD650 drivers. My guess is... that the HD595 drivers sound very similar to the "new version" HD650 drivers.
If I had the "old version" HD650's and didn't want to go to the trouble of swapping phones... I'd likely buy the "new version" drivers and swap them out... or possibly... even the HD595 drivers.
But... all of that assumes... you prefer the Grado / ATH sound to the slow, dark sound of the "old version" HD650's. Some may prefer the sound of the "old version" - as it was a bit smoother, warmer.
Edited by Gradofan2 - 7/25/11 at 2:10pm