Haoleb - I don't think biwiring would help a single driver speaker (headphone).
Asher - The improvement I heard from the Cardas cable replacement was mainly in bass impact and a slightly more forward sound overall - not drastic by any means, but I heard it *shrug*. But then, this is nonsense, so you should ignore what I just said.
Looking back I probably would've been better served by putting that money towards a better source. That's the one area besides switching headphones where I've heard clear (ie not subtle at all) improvements and differences.
