Insane ground loop; anybody wanna try?

the sub is connected to the XLRs via adapter and then by RCA cables.

I wouldn’t be surprised if that turns out to be the key to the problem. While the post I made earlier today in this thread addresses a somewhat different situation, it provides what may very well be relevant background.

The following paragraph in that post may be particularly relevant:

Most XLR-to-RCA (and RCA-to-XLR) adapters short the inverted signal on XLR pin 3 to ground (XLR pin 1), although I believe Cardas can supply adapters on special order that leave pin 3 unconnected. Shorting XLR pin 3 to ground is appropriate when adapting RCA outputs to XLR inputs, and may be appropriate when adapting a transformer-coupled XLR output to an RCA input. However, at best it is poor practice when adapting an actively driven XLR output to an RCA input. And while many and perhaps most components would be able to tolerate that, with some designs shorting that signal to ground may result in major performance issues (see this thread, for example), or conceivably even damage, eventually if not sooner.

The likelihood of a problem arising as a result of shorting an output signal on XLR pin 3 to ground with an adapter figures to generally be greatest when the output impedance of the circuit providing the signal is low.  That is the case with your Capri preamp (80 ohms balanced output impedance, presumably corresponding to 40 ohms for each of the two signals in the balanced signal pair).  In the case of the thread linked to in the paragraph I quoted above the result was buzzing, but hum seems very conceivable as well.

Regards,
-- Al

+1 to what Al sez. ^^^^^^^^^

also:

No dimmers, LEDs, TVs on the same circuit.

These devices can leak noise to neutral and ground, which are bonded together at your service panel.  You'll want to defeat those devices to make sure they're not contributing to the problem.

If I understand when only the pre is connected to the amps you have the hum. Have you tried listing the ground on the pre only? Another thing I would try to further what cleeds is saying is to turn off all circuit breakers in the house except the one for your system to insure it's nothing else in your home (yep you'll have to reset all you digital clocks). If the hum is gone with all the other breakers off add everything back one at a time. If still no hum turn on the breakers one at a time until the hum comes back. Then turn off that breaker and continue to turn the rest of the breakers only to insure it's not from multiple locations.

Also if you have a volt meter measure the voltage between neutral and ground. I get 0.15 VAC. I would start when all your breakers are off except at the system with nothing connected. Keep measuring as you turn on the breakers too.

Not too sure if this will help since you said you unplugged the sub.  My hum problem was coming from the Sunfire True Sub.  Try to carefully tighten up all the nuts that make ground contact to the plate on the amp. Especially the contour and volume pot nuts. I now these hums can drive you crazy!  Good luck!

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!