This is what I know, because I did it in my system, in my house, and heard it with my ears.
I had 2 dedicated lines pulled from a new box, with a separate line at the panel. At the same time I installed 2 high quality audiophile receptacles. After testing and comparing 8 or 9, I settled on Furutech GTX-D(G) and Oyaide R-1 as the ones I liked best.....though the Acme Audio Labs silver plated/cryo/CFC coated are excellent for $75.
Doing both of those things made a big sound difference: quieter noise floor, more top end air, bass was better defined and meatier, trailing notes more noticeable, soundstage was wider if not taller and deeper yet.
Upgrading cables did the same, just more of it in the end. Some did not make any difference, some made a difference that I did not like, and some were noticeable improvements.....but I did learn that cables and gear have a relationship that I can't explain, the same cable that was "flabby" on my tube amps work great on my solid state amp.
Another note, as I've upgraded amps, preamps, speakers, CD transports, etc; the more of the benefit of power and cables are notieceable