Kevin,
If reversing the AC plug doesn't cure the problem, check the ground (or have the cable guy check it) on the cable-TV feed. This is often a source of hum and ground loops, sometimes requiring a copper rod driven into the earth outside your house and a separate ground wire at the cable junction box.
I had that situation with a house in Toronto, and only got rid of the cable-TV induced ground loop by having the cable serviceman install an earth ground outside my house. He also cleaned and sealed all the connections on the cable feed from the line behind the house.
There is another way to isolate a cable feed, using two 75-ohm/300-ohm line transformers end to end, but your problem may not be sourced to the cable TV.
Regards,
