I'm currently using a cable I made myself for my HD 600. I used Mogami 2534 Neglex Quad Microphone Cables and a Neutrik 1/4th connector. Unfortunately after about 2 years the soldering on the right headphone connector is giving way and the audio cuts a little. I would repair it but the Cardas connectors for the Sennheiser 600s are quite difficult to work with and I think I'm more inclined to just purchase new cables.
I was thinking of just picking up the stock HD 650 cables since they are supposed to be slightly thicker and improved over the 600s but I thought I'd check here first. Are there some relatively inexpensive after market options which are good values for the HD 600?