[Pale Rider]

I think progro said it well. Personally, I believe that "everything matters," in the quantum sense. Whether we can hear it may be debatable, and there are plenty of tests that folks flunk. When I started out in audio in my early teens, I had trouble hearing some very obvious things. It was time and exposure, and the learning that came with them, that taught me what to listen for in the first place. As I have gotten older, and had some tinnitus start up on me, I know there are things I no longer hear. But there are things I hear just fine, such as the effect of breaking or burning in[yes, I know they are different terms] some equipment. I also know there are differences in conductivity of different materials, that there are differences arising from equipment and cable matching, etc. To me, it makes sense to do my best job to match my stuff up well, on the theory that it contributes to the greatest possible fidelity to the original signal, and that it will give me the opportunity to hear the most that my budget affords.

It's also fun, and as gadgetman notes above, there is a psychological factor as well. Right now, I am waiting on a new set of Grover Huffman balanced ICs, and half the fun--well, maybe a little less--will be in seeing the construction quality, and trying to see if I can hear any difference from my other cables. In one sense, that's the same reason I am putting together a balanced system, to do it, and to see what I can hear.