Mixing AES/RCA or balanced/unbalanced causes this stuff too, especially in big chain on a bunch of circuits. Audio pro told me this about my 2 10-2 lines that this is an issue in cases. Balanced end puts pressure on the coax end and the tube amp picks it up, swapping interconnects, optical and different things can help too if there is this issue in the chain and reduce or fix it completely. This applies to big chains particularly, I never saw these issues with smaller chains but I can have up to 5 DACs/5AMPs+ on 3 circuits and it can do it if stuff isn't separated and can't flow out right. The grid is unhappy, clogged grounds, and resetting it will get rid of it in cases too. Grounds just get trapped and no where to go slight electrical imbalance inside the chain is a big issue then. Electricity is very fast and can't find a way out in the mess of things. Buzzs/hums from balanced end will hit unbalanced end of the chain and you can turn off equipment and it will jump to the next one if it's bad enough, it's quite funny actually to me.
Sounds like you will be able to find a way to get it to a good enough place. It's certainly lots of trial and error and despair at times! Interestingly I've seen it hum before I turned everything off, cooled it all down completely, shut off the grid and the hum never came back for a week plugged in exactly the same as well. I worried the other day when my tube amp had a tiny noise in it that there was an issue and I cooled it all off and it has not come back for several days. Tube amps are extra freaky about this issues for sure, they get hot they can do weird things so can other equipment when I run it for 8+ hours type of thing. I use 2 RCA sets to my tube amp so if there is an issue it will usually surface for me and I have to fix it which has had me running around in circles a number of times. All part of it I say
Seems like the topic of bad power, grounds, hums really need a dedicated thread for this. So many need to discuss this stuff. Trying not to be off-topic here completely but it's generally in the mix of power conditioning what people are attempting to deal with as much as it is to get lower noise floor ect.