Hmmm. This sounds like some other form of interference other than the 50 Hz mains hum we get. As far as I can see there are 2 potential culprits: either other appliances connected nearby are sending noise into your mains system or there is an appliance nearby that is emitting magnetic or radio noise which is then getting into your amp. I find that if I turn the TV on when I'm playing, the hum from the amp is horrendous. Try turning as many other appliances in your house off sometime and then play through the noisy plug and see what happens.
edit:
Alan Ratcliffe wrote:
Sounds like you have dirty power - something on the same power circuit as your wall socket. Pool pumps, fridges, dimmer switches and a whole host of other domestic electronics introduce noise into the power.
A line conditioner will sort it out - basically a mains line filter with added protection and smoothing. Plug it into the power and all your audio equipment into that.
+1 :-[
