I can give some comparisons between the Zu and equinox cables with the HD650.
First off with the equinox I could hear(or not hear I guess) a lack of upper freq. compared to the HD600. With the Zu this is not so much the case. The phones seem to fill out the upper end much better with the Zu cable compared to the equinox. The sound with the Zu is IMO a much cleaner, tighter, clearer, and more focused sound than with the equinox. This is with both headphones. I love the bass on the equinox with the HD650, and to me it is a much more powerful sound than with the Zu cable.(although the bass is not lacking and go's as deep with the Zu)
The bass is much more forward and seems louder on every recording with the equinox on the HD650. I'm thinking since I don't have many hours with the Zu yet(50-60) it will increase bass a little. If it stays just like it is I'll be happy though.
I've talked to Adam at Zu and he says the fit problem is worked out. If you get a pr. that don't fit, They are the BEST co. I've delt with as far as customer service go's. Mine didn't fit and he sent me another pr. They fit the HD600's but were VERY tight with the HD650. I used a metal emery board and they are in there now, and easy to switch back and forth.
One strange point about the two cables is with the HD600 the bass seems a little better with the Zu. Just the opposite of the HD650. Overall I much prefer the sound of the Zu over the equinox with the 650. It keeps the better bass than the 600 and gives back much of the upper freq. that at first I thought was missing. The Zu lets mucho detail through with always sounding musical. I prefer both cables over the Cardas with the 600. Hope this helps some.