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

One firm noted for early power supply work is NAIM

Bob - yes :-) 

RT 60

or wander randomly.....

true fast is all the drivers start in the right direction , see the impulse response test for loudspeakers. The flat earth random discovery society is anti meter and anti laser. They are no substitute for listening but they are valuable tools. We don’t attain black backgrounds in the recording studio by letting environmental noise in. 

redwooodaudio B and K calibrated mic into Studio Six on an ipad. The trusty and hyper durable Radio Shack analog meter is quite good for low frequency work. Yes dedicated line is wonderful 

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...