[Arran]

I should point out that one of the real winners for RF noise are cheap switch mode power supplies, and even some that are not so cheap like computer power supplies. I had an old computer that ruined AM reception when it was one, not nearly as bad as one of the old DVD players I had though, that would completely destroy AM reception even when it was off. In terms of wall warts the new cell phone charger ones are usually switch mode, and those can cause hash, the old style "wall warts" with a linear power supply, with a proper power transformer, didn't cause that many problems. Another suspect can be some power supplies with a capacitive dropper, but those are more localized for interference such as if the radio is plugged into the same outlet as one, not over the whole house. You were wondering about whether the outlet may have been wired wrong, well it could have, but not so much with the ground prong as with having the hot and neutral wires reversed, 
Regards
Arran