You are getting the hum from the amplifier or the speakers? If you are getting it from the amplifier, it may be a fan or a transformer hum (I am assuming you need to get pretty close to the amp to hear it).
FWIW, I had a 3-way light in my listening room and somehow the wrong type of lightbuld was put into it. Whenever the light was turned to dim or moderate, I got a hum out of my amp. At full bright or off, I got no hum. I unplugged the light until I got a new "correct" light bulb and all is good now. If you have a dimmer switch nearby, check this as well. If all the way off, does the hum go away? Same with other demanding appliances as referenced by Mitch2.
Typically a ground loop hum is going to be exhibited through the speakers and quite noticeable. What happens if the preamp isn't turned on? Does the home disappear?
FWIW, I had a 3-way light in my listening room and somehow the wrong type of lightbuld was put into it. Whenever the light was turned to dim or moderate, I got a hum out of my amp. At full bright or off, I got no hum. I unplugged the light until I got a new "correct" light bulb and all is good now. If you have a dimmer switch nearby, check this as well. If all the way off, does the hum go away? Same with other demanding appliances as referenced by Mitch2.
Typically a ground loop hum is going to be exhibited through the speakers and quite noticeable. What happens if the preamp isn't turned on? Does the home disappear?