personally, i think the only way a new cable is going to make a difference is if it has a substantially different impedance than the original. this is, of course, assuming the original cable has enough integrity and shielding to carry the full, unadulterated signal to the transducers. an additional 75 ohms certainly makes a large difference with the ER-4P, the KSC-50's and the CD3000's. i'm confident that i could successfully differentiate the phones with and without the P->S adapter in a double blind test. the sound quality change is more about the voltage and current requirements than any inherent material "musicality", etc, though.
assuming that all the supposed cable material changes are real, and not merely an artifact of human placebo effect, how is changing the headphone cable going to alter the sound when the wires used internally in the phones, for instance with the Sennheiser HD600, is unchanged? surely the thin wires connecting the cable jack to the actual drivers would negate any other change.
of course, how the cable material, independent of the impedance, would change the quality of a signal is beyond me, so i think it's a moot point anyway. the basic properties of the electrical signal being carried in the cable are well understood, and i've never met an electrical engineer or physicist who has done anything other than laugh at the idea of different materials making changes in the signal. lest that sounds flippant, i try to ask every electrical engineer or physicist i get a chance to talk to the cable question, which has been about a dozen of each profession, respectively.
these unanimous answers, combined with scientific blind tests where cables of similar impedance but different materials have proved to be remarkably hard to distinguish despite the differences claimed, lead me to doubt that the cable materials make much of a difference. unfortunately these kind of tests have rarely been attempted, as few are willing to put their product or ears up against a true scientific test. this is pretty much the same reaction that is given by "dowsers" and "psychics", and other proponents of non evidence based phenomena, which may tell something about the general quality of the claims. as an aside, i have some experience in designing blind tests and i would be more than happy to help anyone design a proper double blind procedure.