I am not sure I can help, but it seems clear to me you do not have a ground loop hum at all since you would hear this through the speakers. The hum you do have appears to be an unhappy transformer. This might be caused by a number of things. Deterioration of the transformer, a fault in the unit causing the transformer to draw more current than it was designed to, low voltage coming out of the wall socket, interference from another electrical field (such as if you have placed another component too close - like underneath or on top), or interference from another appliance in your house or in the neighbourhood - any of the above, and probably some others too. This may give you some things to check. Also try plugging the unit into a wall socket in another room, or a friend's house. Failing that, take it to a technician.
Help identifying a ground loop hum
Ok, I'm talking basic--I now wouldn't know one kind of humm from another if it bit me in the face. I'll describe what I've got, someone please tell me what it is... About 50% of the time that the pre-amp is on, there is an audible, electronic hum that comes directly from its power supply. The power supply hums--not all of the time, sometimes at first, sometimes only later, sometimes not at all--but it defintely hums. Is this a ground loop hum? Is it just sporadically noisey, and them's the breaks? Is there a problem with the pre-amp? Many thanks.