It's not clear to me why it's not straightforward to test these hypotheses. There are apparently plenty of hum sufferers out there, and it sounds like many would be happy to help with any testing that might isolate the source. There are also plenty of anechoic chambers and Faraday cages available. Seems like it would be relatively easy to test the two prevalent theories. You may not be able to pinpoint the exact source, but you could at least tell whether the hum in a given region was acoustic or electromagnetic in origin.