Power Cables diminishing returns

The best diminishing return has stock power cable. It's usually included with component at no extra cost.

Suggest you buy used, if possible. Also, connectors make an enormous difference -- avoid brass and nickel. Duplex outlet makes a huge difference -- Oyaide R1 seems to be the best affordable outlet -- as usual, avoid brass and nickel. Chassis IEC inlet makes a huge difference -- avoid brass and nickel. Without good interfaces, you'll never hear the goodness of a good cable.

Lastly, plating changes the sound. Rhodium, silver, gold, platinum and palladium each have a sonic signature.

In double blind testing audio reviewers could not tell the difference between PC's that were $50 and those at 8K. Several such studies have been conducted --- all with the same results. This would suggest that the "perceived" differences that a cable owner hears is likely rationalization for paying what they paid (or some other psychological reason for the perceived differences). Perhaps there are "real" differences, albeit subtle, in certain systems with fairly unusual system synergies --- but in controlled settings, no one has been able to double-blind distinguish between PC's.

That said, interconnects and speaker cabling is indeed quite a different story --- there the differences can be fairly large. But power cords no. I talked recently to Frank Van Alstine on power cords and he laughed at the notion that people spend over $50-70 for a PC... said, from an engineering standpoint, that anything more is a waste of $.

Well they must be stone deaf then, I'm 78 and can EASILY hear differences.

It's tiresome listening to the Flat Earth Society explain what a human being hears in terms of math and Newtonian psychics.
A human is a miracle of integrated being that hears with its ears, eyes, brain, CNS ,memory, conscious cells in its stomach and many other ways we know nothing about. Not to mention its soul. Of course if you can't see something you have trouble discerning, as you do with seeing if you can't hear.
Conductors in their 80's who can't hear a 5kz tone, routinely correct players playing 10hz tones!

We humans are several orders of magnitude beyond explanation
by Quantum means, much less by some fool running a double-blind "test".
There are more things on heaven and earth than are dreamed of in your tests , Horatio.

One of them is when you leave the mystical out of the musical equation I don't know what answer you may get, but I do know it will be wrong.