Thank you Glissando for your very informative post.
When I turned on my rig and it sounded much better, although I hadn't done anything to bring this about, I put 2+2 together and came up with the answer; the quiet grid that resulted from the COVID 19 "Lock-down".
David, since you're good at finding answers to obscure questions, maybe you can find the answer to this question; I live in a quiet neighborhood, so I know it's not the neighbors affecting my AC, but industrial activity miles away; the question is, how far away can industrial activity affect your AC?
As I explained before, I down-load LP's to hard-drive, and listen to my "play-list" from the computer at my leisure. After I noticed the new recordings sounded better than the old, that confirmed a number of things; not only was the computer functioning better, but everything else as well; power supply for phono-pre plus everything else that has a power supply.
There are millions of people who are benefiting from this "lock-down" and don't even know it. Now that many are going back to normal, that might be past tense; it came and went, and they didn't even know it. I on the other hand, have it recorded on my "external hard-drive", which means that if the computer turns to junk, I have it all in a little box that I can take with me