@rooze
I have noticed this with all of my systems, starting at 15 years old with a "big time" Radio Shack rig, their most expensive system (my old man thought I was nuts and fought me about it).
Even with that crappy receiver (STA-140) and Optimus 5 speakers I could tell that difference after a few hours of listening. That was 55 years ago.
I really don't think it had anything to do with quality electric. Music lights up certain circuits in our brains and quiets others. So, as you relax and release from the trauma (yes TRAUMA) of the day the music becomes better and better.
Night time is better, but I think has more to do with circadian rhythms. The less light entering your eyes also quiets circuits as your body prepares for sleep.
OK, having said all that, power does make a difference, but not as much as the above. Really crappy power will screw things up no doubt!
YMMV
Regards,
barts