Diagnosing and fixing transformer hum

Ping, I’m not sure if this is helpful but I would use a circuit tester on your home’s outlets to see if they are wired properly. I also would call an electrician to see if your service panel is setup correctly, grounded, breakers ok etc. sorry you’re having this issue. when you solve this would you let us know what you discovered and did? Finally, what did your amp mfg. have to say about your issues?

If it's not DC offset, it may be that the transformer(s) are simply loose enough to vibrate. Have you tried loosening/tightening mountings bolts? 

With one particular amp I owned, and after trying everything possible, the only culprit remaining was delamination "rattle" from loose windings over time. Sometimes the transformer is simply bad...and an electrician isn't going to help anything.  It's the only amp that's ever hummed in my setup at the same house - tube or SS.  

The house was built in 1941. Even light fittings seem to hum.

Its really hard to find explanations of DC offset that don't make my eyes blur and head ache. Briefly it can be any fault in the electrical system- any connection from the panel to the light or appliance running on the circuit. A home wired in 1941 has had 70 years to develop faults.

The easy fix would be a hum buster. Granted you already tried one so a bit of a long shot but worth it compared to the alternative. Which is starting at the panel to take everything apart and clean and improve mechanically (I'm talking like emery or sand paper wire ends, bus bars, etc) and/or treat the whole thing with TC. Then continue on to do this for every wire and replacing every outlet and switch throughout the house.

The wire is fine its the connections that oxidize and cause trouble. Can't guarantee this will totally eliminate your DC offset hum but it will for sure have your system sounding a whole lot better. 

btw if that sounds like a lot, I took my panel apart all the way down to the bare bus bars. Those little brackets that screw into the copper bus bars? The ones the breakers clip onto? I removed all those, cleaned, and treated with TC. If I can do it you can too.

@tuberist 

Good thought. The DC blocker also includes a tester, thanks. A couple of outlets are wired incorrectly, but hum is there when using the outlets with no problems. I will get an electrician in to do further checks and fix such issues, for safety if nothing else. 

NoRom and 3EP: 

yes, I was thinking that maybe the house move loosened things. Unfortunately, the way the unit is designed, it looks like getting to the mounting bolts of the transformers means removing the circuit boards underneath, which I don’t have the tools for. The dealer is in a different state, and we’re on lockdown. So I will count myself lucky for having luxurious problems and wait a while to have it checked out. 

Miller 

I will get someone in to do some safety checks and tend to a few issues I can already observe. A local dealer has recommended his electrician as being audiophile sympathetic. When he’s there (and confirmed nothing is live!) I’ll polish connections like you suggest. Even though switching off all the other circuits didn’t fix the transformer hum, it did lower the hum at the speakers, which suggests some combination of ground loops, EMI, etc. So yes, point taken.

BTW, what do you mean by TC? I presume not “tender care”? 
Ping

You have a DMM, 120 on the button. usually takes care of transformer noise. Older gear 117 or less, as low as you can go without transformer noise. Have access to a variac, and a DMM, might just tell the whole story,  1941 construction, knob and tube and maybe lath and plaster. Noisy stuff can really echo in a room too. no carpets lath and plaster, tall ceilings, just a thought. 

millercarbon says clean, look, sand, tighten, good place to start and maybe finish. Take a peak inside your main at night when the noise is going on, LOOK. might see something. Be careful.  

Regards

@oldhvymec might be on to something. If you have a variac, try a test adjusting the voltage from 100Vac to line and see if that changes it. You have a 300B tube amp. Try a different rectifier in case there is a load issue causing a problem. Bad caps can also cause an issue like this.