curious about your feedback (no pun intended) here ....
both of you have indicated that (paraphrasing) a change in line voltage could be the rationale as to the reason for a blacker background or perceived improvement. In my case I currently have virtually no "noise floor" (i.e. very black, etc.). Very happy in this regard! I am using a Panamax MR4300. That unit has a voltage level monitor. My electricity service is provided by the "incorporated village" of Freeport and is known (rated) to be among the best overall electrical service in the US.
My MR4300 consistently (when on or off and whether or not I'm listening to music) indicates the voltage is varying. It has measured as low as 116 and as high as 123. Regardless of the reading, I am unable to discern any audible difference. So my point in soliciting your feedback is to understand what degree of change you are speaking of, etc. Any other input is also appreciated.