Diagnosing and fixing transformer hum

Moved into a house two months ago and have (finally) set up the system in the living room. (The plan is to turn the unfinished basement into a listening room.) Amplification is all tube, with a push-pull 20w 300b integrated amp. The transformers in the integrated hum loudly; there was only a very mild hum when we were in our condo, quite tolerable when sitting close to the amp.  The house was built in 1941. Even light fittings seem to hum. 
Things I have tried:
1. Different wall sockets
2. Switching off appliances 
3. Turning off all other circuits from the electrical panel except for the wall socket in question. 4. Using an Emotiva CX2 DC blockerIn all cases, no improvement. Any suggestions to diagnose, isolate, and fix?


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. 

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”? 
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.  

@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.