Are you thinking of upgrading to the Cardas replacements? I haven't convinced myself to pull the trigger on that one, but given the resolution level of the Sennheiser HD 580/600/650 series, I'd be surprised if you *didn't* hear an improvement. The Cardas is also supposed to be more rugged; George designed the cable for his daughter who's a DJ.
Once you get into that money, maybe you should look at http://www.headphone.com for their balanced headphone option
Once you get into that money, maybe you should look at http://www.headphone.com for their balanced headphone option