As for why power cords & signal cables make a difference, it has to do with our noisy power grid and all the EM interference we have in the air these days, along with the crappy power supplies & grounding found in many pieces of audio gear, not to mention questionable circuit design. Those interconnects end up working as filters to help keep noise out of the circuits so that the audio components can perform as designed. If we had perfect power supply, grounding, shielding, and circuit design, then power cords & signal cables wouldn't make a difference.
This controversy over audio cables has been raging for almost 40 years. It’s unlikely that we’re going to add anything here that hasn’t been said many times. A search of the internet will easily produce results to support whichever position you hold.
Allen, I’m sure you can cite many sources to support your assertions, but they have also been refuted many times. For example, here is a quote from “Audio Critic”:
The assertion “that high-priced speaker cables and interconnects sound better than standard, run-of-the-mill (say, Radio Shack) ones … is a lie that has been exposed, shamed, and refuted over and over again by every genuine authority under the sun …”
http://www.theaudiocritic.com/back_issues/The_Audio_Critic_26_r.pdfI expect you and many others will disagree, but the author backs up his claim. See “The 10 Biggest Lies in Audio” from the link above, as well as Issues No. 16 and No. 17 of “Audio Critic”.
It’s worth repeating, however, that you and others can cite contrary authority. In the end, anyone truly interested in this topic can easily do their own research and decide for themselves who makes the better case.