+1 millercarbon's point about AC noise. OP, if you really want to do a comparison, make sure you do it during the same period -- all during the day vs. all during the evening. If you're comparing evening to daytime, then you're not eliminating the "noise on the line" factor (which may or may not be a factor where you live, but you'd eliminate it by simply controlling the day/time.
Amplifier warm up
I was plalying Boz Scaggs, "Silk Degrees" LP. After side one, I ate dinner. One hour later I played side 2. The second side sounded clearer, more vibrant with better sound stage. I have a Parasound P6 preamp and Parasound Halo A21+ amp. Is it possible that an hour of warm up improved performance?