"But it sounds better at night...."

I don't smoke weed during the day

It’s your mood 98%. 2% of the other stuff. 
At least for me. 59 years of experience starting with a tiny transistor radio and now into mono blocks and Magnepans.

I addressed power at the meter. Made sure everything was optimal without buying into fancy audio conditioners or the like. I do have whole house surge but after making sure the main service panel had no issues, set up a subsystem, tied to the main household ground using a large 10kVa iso transformer from Controlled Power. That feeds a sub panel adjacent to the music room using 4 gauge, and from the local sub panel, pulled 10 gauge dedicated lines.

The system here sounds good. I think I benefit from newer grid infrastructure and even though I'm located very close to downtown Austin, no commercial operations are fed from my transformer. The system sounds the same, day or night. Much better than the power I had in the lower Hudson Valley, north of NY, which was ancient and prone to outages. The Texas grid is suspect though and I'll eventually install a whole house transformer- not for use on the audio system, but just to have power if we black out. 

All power solutions should start at the meter if not before (though unless you have pull with the local power authority, you aren't going to get a dedicated pole transformer). Start at the beginning, rather than adding bandaids. If you are in a condo or apartment you are far more limited. 

Aside from solar radiation, quite real, there is industrial noise on the power lines during working hours. Neighbourhood activity also contributes. Soon after about 10pm when most people have turned off their lights, computers, microwave ovens etc., the lines are more free of noise generated by everyone else's electronic appliances. Even LEDs create line noise, so better to turn off your lights and even leds in your gear.

More usage during the day reduces line voltage too, making sound slightly harsher, less natural.

Having a power regenerator helps heaps, but it is still affected by line noise, surprisingly. This is snake oil for people with cloth ears, but there are genuine benefits from listening at night.

Thanks everyone.  Think I can distill it all down:

1. Try a power regenerator after recovering from today's material financial outlay (new ARC preamp).

2. Listen to music at night, with beer and darkened room, and enjoy.

Part 2 can start immediately!