Insane ground loop; anybody wanna try?

Again, thanks to everybody for taking the time. Here are new data as of this morning:

@cleeds I never think of my video stuff as a separate system, but it obviously is. I ran the preamp through a Meridian 551 integrated amp and then to a pair of Totems. All but dead silent, so it doesn't seem to be a noise problem with the preamp itself.

@almarg I was so hopeful it was the XLR/RCA adaptors, as Al suggested (the easiest fix, after all). Alas, unplugging them changed nothing. While I was at it, I switched the amp runs from the RCAs  to the XLR/adapter outputs. Again, no change.

@xti16 I did try lifting preamp ground only, with and without using the Hum-X on the preamp. Slight changes, nothing significant. I'm resisting the circuit-breaker dance for two reasons: first, see data below. Other circuits don't seem compromised; and, second, the breaker box is three stories down and while I'm in decent shape, I am no longer 25 years old. ;)

Okay, here are some more data.

Plugged the amp and preamp in two other parts of the house, using the same cabling each time. Here's what I found:

Location 1: Recent construction, new electrical service 2004.
Wiring is correct.
No hum at all.
Neutral/ground voltage: 0.1VAC

Location 2: Original construction, 1936 wiring but service upgraded, probably 1990s. Not the same circuit as #3 below.
Wiring is correct.
Barely audible (acceptable) hum
Neutral/ground voltage: 0.0VAC
Hot/ground voltage: 0.8VAC
Yes, those are right; I measured several times and re-checked wiring

Location 3: Listening room
Wiring is correct
Hummmmmmmmmmmming bird Farms, may we help you?
Hot/ground voltage: 0.9VAC

I'm now wondering if it's something to do with the circuit that feeds the listening room — but, again, that's just logical speculation, blissfully free from the constraints of actual circuit knowledge.

Anybody see anything helpful here? Thanks again!

Oh, and @mcroth — thanks for the suggestion, but you're right, when the sub is totally unplugged the hum is diminished slightly, but still pretty audible from several feet. I appreciate your raising the idea!

Hot to Ground 0.8 and 0.9VAC? Looks like an open ground. What's Neutral to Hot?

@xti16 Hell, sorry. That should be neutral/ground in #3 above. Anyway, it’s 124.5 in both spots. Also, the receptacle tester reports normal service in all three locations.

Okay, here’s a little more info. (I work at home. You can see I’m getting a lot done today. LOL.)

I have a SS amp with balanced inputs as well. When I substitute it for the Quicksilvers, the hum disappears, even with the sub connected. This is true whether I connect the SS amp directly to the XLR outputs or to the RCA outputs via RCA-to-XLR cables.

This suggests some kind of issue between the Rowland preamp and the Quicksilvers, yes? It also suggests that the AC power by itself isn’t really a problem. But whaddo I know? Any ideas?

And to answer the obvious question: Yes, the Quicksilvers sound about a zillion times better, so keeping the SS amp instead isn’t really an option.