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

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. 

RT 60

or wander randomly.....

RT 60

@tomic601 ,
Is this what you are referring to?
https://www.nti-audio.com/en/applications/room-building-acoustics/reverberation-time-rt60-measuremen...
Bob

@tvad what kinds of mods did you undertake?

The first notable improvement in blackening background in my system came from components with robust power supplies. These changes initially came from having early source components modified with significant improvements made to power supplies. The background became blacker. No question. Improvements were easily heard.