You can see when a post was edited by looking at the bottom.
But it's a long thread so easy to miss details.
Did some more playing around today. Discovered I really have four problems, not one.
1) The outlet behind the audio cabinet is no good for audio. That's why I got hum from both AVRs. I was surprised to learn by playing with my panel that the outlet actually isn't on the "living room" circuit. It's on the "dining room" circuit. Same as the dimmer and the thermostat on the other side of that wall. So, I'm definitely calling an electrician to change that. Now, the question is, do I suggest the electrician put the audio equipment on the same 15A circuit as the TV, or on the same 20A circuit as the subwoofer amp (basically right under the living room)?
-Easy access for both. The 15A outlets are separated by, inter alia, a double doorway, but there is easy basement access. The 20A circuit is right below the living room.
-I also have a UPS there, because my cable modem and router (Apple Time Capsule) are plugged in there.
-I run balanced from miniDSP to sub amp, so I could use disconnect the shield at the miniDSP per the Rane app note. Both miniDSP and amp use Phoenix connectors so I wouldn't even need to open up a cable to do so.
-There would be a lot of amplifier on the circuit. Mains amp is Class AB and rated 200W/8Ω/7ch driven, sub amp is Class D and rated 1kW/4Ω/4ch. That said, I ran both amps and all of my other A/V stuff from the same 15A circuit in my old loft without any breaker-tripping issue. And that living room was considerably larger than my current one, because the living/dining/kitchen was one large space.
2) There's a ground loop between my TV (or my HDbaseT baluns) and the audio gear. That's why I thought there was hum from the basement circuit. If I disconnect either end of the HDbaseT transmitter (either the ethernet or HDMI cords) the hum goes away on that circuit. It also goes away if I put a cheater plug on the TV. My TV is over 5' off the ground, plugs in to an outlet directly behind it, and has a plastic case, So unless someone can give me a good reason not to use a cheater plug there, I'm less bothered by it than I am with a giant metal-cased amp at toddler level.
3) Ground loop on the incoming cable. I'll be getting an isolation transformer for that by Tuesday. Interestingly, with an extension cord running to the TV outlet, when I plugged the cable into my power strip/UPS, the hum got much much worse.
4) Cable-box dropouts are still a thing when the HVAC cranks on, no matter where I plug the system in. I'm wondering if rerouting the coax might fix that. Cable box and AppleTV do not drop out when the amp relays kick on when the system is plugged in downstairs or to the TV outlet, but the HVAC does it regardless. I haven't had a chance to the load test Andy discussed above, but it's next on deck.
So, I'm thinking the second problem I solve with a cheater plug on the TV, and the third I solve with a catv isolation transformer. The first and fourth I solve by engaging the services of a licensed electrician. Sound like a good plan?