The key here is who is listening. Many, many people claim to hear a difference in audio cables, myself included. If you can't hear a difference in audio equipment, then the upgrade is a waste of money.
Generally speaking, it is impossible to prove that "you can't measure a difference," but it is possible to prove that a certain group of people can consistently detect a difference. If I run 1,000 experiments trying to show that people can hear speaker cable differences, and none of them succeed, then I have proven nothing. Yet with a single successful experiment I could prove that at least some people could detect the difference.
This is purely a matter of statistical and scientific methodology, particularly regarding the "null hypothesis," and has nothing to do with speaker cables or audio equipment.
-Cooper
