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!