“Black background” — What really contributes to this phenomenon?

I like that answer.

Everything helps.

Listen a night and turn lights off.  Boom. 

Power? Sources? Turntable? Amps? Cables? Tweaks?  Vibration control? Any of these in particular?

No. Not really. It's all of them, and more.

Oh and, fixed it for you.

room, noise, appliances, AC, windows, etc....think BIGGER than just the stereo and AC power...few do..

double core door, weatherseal, etc...

and a good spl meter will help you establish a baseline noise floor, so few audiophiles own or use them...