RCA Cable Quality

Good for you (and your wallet).  Frankly if I couldn’t hear significant differences between interconnects I’d be concerned about my system’s transparency or my ears, or both.  In my system interconnects make a bigger difference than speaker cables (although my speakers are an easy load that I think affects that), and I think the digital cable makes at least as much difference as the interconnects if not more.  Just my experience in my system and obviously will vary. 

I have never heard any differences between cables. I did buy five pairs of Audio Quest Silver Extremes for $450. They look nice!

I've had MIT shogun RCA cables since the ninities  Can even remember what I payed for them But what I really like about them is their high quality locking banana connectors  They grib like nobody's business 

Same experience as OP. Tried a bunch (on loan from local hi-end shop), not an iota of difference. Same for speaker cables (AWG14 vs 5–6). So as long as it has mechanically decent connectors (e.g  Blue Jeans), I'm good. Speaker is bare wire, of course.

If someone questions quality of my components, look at my virtual system. Not stratospheric, but not entry-level either. So going from entry to something "better" has best shot to show *some* differences, but it doesn't. Tried it enough times not to bother anymore; remember definition of insanity.

As I mentioned before, differences in recording styles are MUCH greater, so "optimizing" cables is a fools errand. Or are the cable guys changing cables depending on which recording they play? LOL!

I think it is a fair point to acknowledge that things like cables and wire fall under the "law of diminishing returns"... I DO believe that there is something positive to be gained from almost any decent system by upgrading past the 'freebie' RCA interconnects that come with some equipment and the 'discount' bulk speaker wire towards products that are at least somewhat higher quality. I myself swapped out those cheapo free interconnects with some modestly priced (e.g., $20 each range) interconnects and absolutely noticed a reduced noise floor and especially a change in the signal from my turntable. I also upgraded speaker cabling from bulk 16 gauge to some SVS shielded 14 gauge and again, most notable difference was less background noise during quiet music passages, etc.

I also recognize that there is the POSSIBILITY of greater sonic differences  / performance when utilizing equipment that is more discerning (read that as "higher end") than the equipment I have. I also believe that it is entirely possible that noted differences above a certain threshold may be entirely listener bias and not anything measurably superior.

Lastly, I would add that any hearable difference likely decreases in both scope and range as you move up the cost ladder... and you likely will end up chasing truly almost intangible improvement with increasingly larger dollar purchases...