Ha, I never have issue of hearing louder or quieter. Always seems the same at the same setting.
But anyway, my suggestion is very basic, if all sources ( CD, streaming, radio etc.) have the same loudness, then the source is not the problem. Left and Right channels are separate circuit, it's hard to imagine both get quieter at the same time. If OP feel both sides got quieter, then it's more likely it's his ears. I just don't want to make assumption at this point.
Own designed power amp, own designed preamp, JM LAB 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.