I haven't read all the responses so far. I had a similar affliction and it turned out to be a loose connection from the house wiring to an outlet that wasn't in use but on the same circuit leg.
Plug the entire system into a power strip and the strip into an outlet then try lifting the ground with a cheater plug. listen for any changes.
You should hear some hiss with your ear right next to the tweeter which should dissipate within one to three feet.
Plug the entire system into a power strip and the strip into an outlet then try lifting the ground with a cheater plug. listen for any changes.
You should hear some hiss with your ear right next to the tweeter which should dissipate within one to three feet.