It is hard to troubleshoot on line like this, I have to get my hands on it and would be a lot easier.
The thing that alarmed me is when you said you cleaned up the connections and the noise got much lower, that pointed to the connection the cable.
BUT then you said the noise got worst after warming up for 30mins, that threw me off, bad connection usually don't do that. That's why I asked you to jiggle the wires and see what happened.
I am glad you get to the bottom. Those RCA connectors can be pretty bad, when I buy RCA cables, as long as it's the thicker type, I don't buy into special metal, directional signal and all the fancy stuffs, but I do pay attention to the connectors, I bought a lot of RCA cables from Amazon, some have really loose connectors and it's no good. Don't spend a lot of money on those RCA cables, just make sure it's thicker type like RG-59 type.
Own designed power amps, own designed preamps, JM LAB Focal Alto Utopia and Spectral 913.1 speakers, Rythmik F12SE sub.
Not hooked up: Nakamichi Stasis PA-7 power amp, Velodyne VA1210 sub, Kef Reference Series center, Kef Bookshelf speaker, Monitor Audio bookshelf speaker, Infinity rear speakers. Acurus 3X200W amp.