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

@bigkidz adding an ac filter choke? Please elaborate. 

I like that answer.

@tomic601 what spl meter do you use? I like that suggestion.

Overall, I think it has been the dedicated line that helped the most so far. Unfortunately, there is a refrigerator 25 ft from the listening position that ain't going anywhere... 

@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. 

The resolution of the components and cables matters a lot too. Not only to reduce noise, but to being transparent enough to discern the micro detail and air in the recording. Getting the black background and not wasting it with lossy signal path is a real investment!