In my experience on Head-Fi,the Zu Mobius is generally recommended as the best overall replacement cable for the HD 650's. It is very expensive in relation to most of the others, however (believe it runs about $250 for a 3 meter length - check Zu website to confirm). After the Zu, the Equinox and Silver Dragon have many proponents. These are not as expensive as the Zu, but the Equinox has a very long waiting period at the present (16 weeks) if you want to order one. You should also consider Larry at Headphile.com's replacement cable for the HD 650. I have talked to people who have used them, and they are considered to achieve within 85%-90% of the Zu's performance for about half the cost. He's also a great guy and nice to deal with. Another cable replacement option is the Oehlback cable offered from Meier Audio. This cable is generally regarded as only a modest improvement over the stock cable, but it is fairly inexpensive ($60). Hope that helps.
