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?

pingvin