Has anyone updated Senn HD650 cable?

Yes, I did: Revelation Audio Labs cable. Some improvement was gained, like better resolution and increased dynamics. It's a much higher quality cable than what Sennheiser provides and worth the investment if you're a serious listener.

Yes, I tried the Cardas which was an improvement but found the Moon Audio Blue Dragon V2, and later, the Black Dragon significantly better. The Cardas, like many of their wiring has a lower midrange, upper bass hump. The Moon cables were more balanced and still more revealing.

...and the Sennheisers don't need the added warmth of the Cardas although it is an improvement in other ways.

Yes - I heard a definate improvement. I am using the Headroom Bockhead amp which is a balanced amp, an Ayre preamp, which is also balanced,and therefore, I wanted a balanced XLR connection for the Sennheisers. The improvement was not at all subtle. I am not a fan of Cardas cables, however, Im leaving these cables attached to the phones.

Check over at www.head-fi.org. Lots of information on which aftermarket cables to try. I have both the HD600 and HD650. I tried several cables, but not all. I've found the Cardas has better synergy with HD600, and the Zu Mobius work better with HD650.

FrankC