Thanks for the info. I just had a chance to briefly listen to the 600s with the Cardas cord. Wow! That is a major league improvement over the stock cord, but it should be for another $150.
Sennheiser replacement cables
It's interesting that the Sennheiser HD580 and 600 use the same replacement cable, h-81435, while the 650 uses the h-92885. The 650 replacement cable is actually a bit less expensive than the other cable, and I've been told it's a better replacement cord for all 3 models. Any thoughts? Thanks