It is rather unfortunate that to avoid fanaticism and confrontation, I have to preface my post by saying this, but I will note that these are my opinions only, backed by what I know about science and my engineering degree, but others may have a different subjective experience.
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That said, no, I do not believe cables make an ounce of difference as far as humanly perceptible sound quality goes. I tend to get premium cables or make my own, but that's only because I prefer the better build quality, a more flexible cable, or because stock cables are often too long (seriously, I'm not in a studio recording my platinum album... I don't need a 10ft+ cable...). Plus, it can be fun to DIY.
But for SQ? Not a chance. Just think about it -- if cables really made such a great difference, why wouldn't the headphone manufacturers include all these newfangled silver, etc cables of 7N+? Or why wouldn't they make their own and sell it at an aftermarket premium? If it actually mattered, isn't failure to do so negligence on their part by gimping the maximum potential of their own product? In a market where the upper echelon manufacturers are separated by only minute differences, why wouldn't they want to squeeze as much out of their headphones as possible?
Alternately, just think about the actual worth of your entire headphones. If you have, say a (once upon a time high-end) mid-fi phone like the HD650, which is roughly $300-350... how can spending that much, or even half that (or even a quarter), be worth it? Wouldn't it better to, I don't know, get a new pair of headphones with that money than to try to squeeze infinitesimal performance out of the existing one?
Third counter point. All chains are bottlenecked by the weakest link. People who insist cables can be bottlenecking your system fail to realize that cables aren't just the wire that's inside. Cables are composed of interconnects, which probably aren't 9.99999999999999999% pure unobtanium and function by mechanical contact rather than a strong electrical contact bridge like solder. So, even if you got the best cable ever, that's not the bottleneck, the interconnects on the cable and your system are. Let's go a step further. What about the solder that binds your cable to the interconnects. Chances are they're some combination of tin and lead. Let's go even further. Your actual amp, DAC, etc. have all kind of components soldered and if there's point-to-point wiring, those wires are probably not as nice as the cable wire itself. 99 times out of 100, it's going to be OFC copper, or worse. Replacing your cable and hoping to get better sound quality, without realizing that there are other electrical bottlenecks, is like if I upgraded my graphics card from a Radeon 7970 to the latest Radeon 7990 but am still running a Pentium 2. The graphics card isn't the issue, and neither are your cables.
I could go one all day about the science about it too, but since people are touchy on about science here, I'll save that for the sound science forum and just present the three arguments above based on logic, not science. There are reasons to get better cables (again, I do!) but sound quality is not one of them.
