How much does the quality of the RCA interconnects running from the power amp to the AVR really make? Are there really any noticeable gains from cheap to mid-level to high end RCAs?

The ones I purchase from Monoprice/Amazon are relatively cheap and that is all you need.

I am running 40+ year old interconnects from ratshack with my actives. They are of various lengths and are like a mess of spaghetti. Different rca cables may measure different but I don't know if the differences are audible.

That's more or less what I was assuming. The only audible difference I've ever experienced was running shielded RCAs in my car which prevented road noise from coming through the speakers.

Head to guitar center. Geez they need your help right now. Cheap and good stuff that is used by the industry.

Amazon Carries Gotham/Mogami RCA cables. They're pretty good. Oh, and 'Best Cables in the World' they're pretty darn good.

Blue Jean's carries Belden and Cardas, I think. Pretty good.

All modestly priced.