how to eliminate air-born RF

Assuming that Edesilva might be on the right track about a very strong electromagnetic field (EMF)expanding and contracting at 60hz frequency from a rogue external power equipment, your speaker cables can pick up that like a wire-loop induction. If your speakers are efficient enough it might be playing the hum all day long without your amp turnon. I can be wrong, but try disconnect your speaker cables and see if the hum is still playing.

I do hope I'm wrong in this intriguing Mooncrikit case. There were many health concerns raised by people who live near sources of EMF recorded by Mark A. Pinsky in his book "The EMF Book" 1995 Warner.

I corresponded with Mooncrikit briefly and from what i can gather, Edisilva seems to be on the right track. It is almost as if there is an external source for the vibration / hum. Only problem is, Mooncrikit has experienced this when the AC mains were disabled, meaning that the power to the entire house was not functional. As such, it looks like Philip's suggestion about an EM field could be coming into play here. I have experienced this problem to what most would consider a "scary" and very visible extent before, but not inside of a house.

As to try and narrow things down, how far is your house from a large pole transformer ? Do you have High Voltage power lines in close proximity to your house ? Is the AC feed into your house in very close proximity to this room ?

To be quite honest, this sounds rather baffling to say the least. Sean
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This is a real shot in the dark, but check to see if you have a thermostacially-controlled vent fan in the gable end of your house. When I moved into a house once I did not know one existed. I keep hearing a damn humm, but only in one room, and only on occasion. Now, if you have truly disconnected power from the main, my point is moot.

yes check your attic for a attic exhaust fan ours is quite noticeable if your near the wall where its mounted into. But killing the power to the entire house should turn its power supply off also? or possibly do you have any of those round attic vents they aren't powered by electricity they work through the heat rising it could need new bearings if you have any.

I am getting dit dat from a ham operater keying somewhere.  I just ordered some material that is supposed to shield from RF.  I have a
SOTA sapphire turntable and Hana low output moving coil on a unipivot  Ultracraft tonearm.  It is a function of where the cartridge is located above the turntable.  It is highest when not above the turntable platter but outboard of the record perimeter the noise  has some varation in volume and stereo channel it is on depending on the location over the turntable.  It does not disappear at high volume but is still apparent over mostly the outer edge of the platter but over the outer inch or two of record grooves.
I have tried different tonearm pipes(the tonearm has changable "pipes" not changable headshells), and tonearm bases.  It is present on a Hana low output moving coil and also on a moving magnet Grado mounted on a second tonearm pipe. Different tone arm cables make a minimal difference in the amount of the noise.   I even tried adding ferrite chokes over my tonearm cables 

Any thoughts?