well, you did all you can do to rule out ground loop. all that is left is dirty power or failed amp. dirty power can be caused something in your home or by the utility. in your house things like CFL/LED lighting, battery chargers, and motors.
CLFs and LEDs can put noise on the line when they are on, dimmers for this lighting can do this even when off.
battery chargers, cell phone adapters, and anything that turns AC to DC wil do the same. but you would need hundreds watts worth to cause trouble, or a few failed adapters. they would be excessively warm to the touch and batterys would take longer to charge. this could also be a symptom of trouble on the utility side.
then there could be a problem with your homes wireing. this would have been giving you trouble from the first day. you eliminated a single circuit fault so if there is trouble it would be at the main panel.
to fix these problems you would have to contact your electric utility, or an electrician to look over your wireing and connections. you could look into picking up a power line conditioner, you can get on at your local Audio/video shop.
or your amp/preamp is failing on your speaker system. any of the above would not fix this