You have already tried many things and gotten a lot of good advice so I will offer a few short items.
First, did David Berning offer you another pre amp? For starters, it would be interesting to see if the problem continues with an identical model. If it doesn’t, then you could make some sort of deal with him about swapping your current one for the replacement.
Second, I don’t know what a cheater plug is, but I have had a few hum problems many years ago. One was solved by a PSAudio device that turned hum into a light (nightlight). It was the size of a match box, attached to the power strip, and the first hum problem that I had disappeared completely. A few years later with changing components a nastier hum wasn’t defeated by this some I bought something called a “Humbuster” from Sweetwater. This required plugging the problem component in to the humbuster and then plugging that into the power strip and was entirely successful. I couldn’t tell if the SQ from the component in question was affected
First, did David Berning offer you another pre amp? For starters, it would be interesting to see if the problem continues with an identical model. If it doesn’t, then you could make some sort of deal with him about swapping your current one for the replacement.
Second, I don’t know what a cheater plug is, but I have had a few hum problems many years ago. One was solved by a PSAudio device that turned hum into a light (nightlight). It was the size of a match box, attached to the power strip, and the first hum problem that I had disappeared completely. A few years later with changing components a nastier hum wasn’t defeated by this some I bought something called a “Humbuster” from Sweetwater. This required plugging the problem component in to the humbuster and then plugging that into the power strip and was entirely successful. I couldn’t tell if the SQ from the component in question was affected