I had this happen with my old condo. I was close to their radio tower though. I agree with the grounding problem with the outlet. Try a different outlet or try to find a different facing for the amp. If you can identify their tower try to find different positions away from it and maybe that would help. I wish I could say that that would get rid of the problem. However, mine persisted to various degrees until I moved.
My stage rig with separates and noise gates did pick up the signal much less than my practice combo though.
Good luck!
My stage rig with separates and noise gates did pick up the signal much less than my practice combo though.
Good luck!