Hey, ya know, we try our damnedness to quantify everything, but maybe we're all just different. I think it sounds more detailed at night, and I'm using a portable source and a battery powered amp.
So it's radiated AC "pollution" maybe? I mean ask anyone that's ever been here, the first thing you miss or notice coming in/going out is that your body resonates at 60Hz... you can feel electricity on your skin, in the air, etc... it's tangible. (mommy, what's a terawatt?)
While that's true, see what I mean? We'll try to use any form of justification as to why, but always after the fact. You can twist anything to fit afterwards though. Does it get easier after you make multi-thousand dollar investments to prove something? Uh.. another thread.
I'm sure AC is both more stable and cleaner at night, even at your local neighborhood transformer... but wouldn't that also depend on your power supply for your gears? So gear with robust supply might be affected less than a cheesier one. Same for cleaner devices. I mean I run off batteries via an on-line UPS, so technically even if I plugged in I shouldn't notice a difference in day and night sessions. It's irrelevant if an add-on device like Jude's balanced power converter would make this even cleaner - it still shouldn't change in quality from daytime to nighttime.
Hmm. I think I'll stick with the original thought re: day vs night. Same thing as closing your eyes while listening, just stronger. I M O