@mclinnguy thanks for your detailed effort to make an objective report on the OP’s system. Based on the OP’s initial post on his, his neighbor’s, his dealer’s observations, and their n your report, I am convinced that his system in his room is resistant to cable changes. I believe that is true based all evidence presented.
This has not been my experience. My room is purpose built and has a dedicated twenty amp breaker and 10AWG AC line that only supplies my audio system. Many but not all cable upgrades have made an audible difference in my system, and not always a positive difference and not always covarying with price.
Earlier in thread it was suggested that a dedicated AC line would help. I highly recommend this step, as everything downstream can be affected by noise in the AC. Maybe this is in fact what the OP has supplying his system, I haven’t scanned the entire thread.
@Mclinnguy noted that there was a long extension cord connecting the system to the wall outlet. I wasn’t sure if this was successfully bypassed. If not, it certainly wouldn’t help. The last cable upgrade I have made was to put a different PC between the wall and my power distributor, and that has turned out to be revelatory.
It has been suggested that the character of the OP’s gear/room combination does not respond to cable changes, and it has been suggested that if the OP is happy with the current sound of their system, then yah(!) the OP can save a ton of money and time fiddling around with expensive wires. I concur.
I suggest if the OP is still cable curious, they go to their dealer and ask them to demo some cables in their shop in a system that the dealer thinks will result in the most dramatic difference in sound reproduction, and make the dealer prove their case in what they consider ideal circumstances. I did this in shopping for my first aftermarket power cable, and the results were not subtle.
YMMV
kn