Lots of possibilities.
First, does the hum go away when you connect the amp to a preamp or other source? If so, then just don't ever turn on the amp without it being connected to something.
Second, does the amp's line cord have a grounded plug (one with three pins instead of two)? Are you using a "cheater plug" to bypass the ground pin? If you are, then DON'T.
Third, get one of those $2 AC circuit testers at Wal Mart and check that your AC plug is wired correctly.
Fourth, are there any lamp dimmers in the same AC circuit as the amp?
In fact, are there any dimmers in the entire house? If so, remove
them, dig a hole in your back yard at least 2' deep, throw them in the hole, cover with dirt, and jump up and down on it until they're packed-in so tightly that they can't escape. Replace with real light switches.
If none of these work, then the problem may be in the amp. Could be bad power supply capacitors, cold solder joints, or one of many other things. But check the easy stuff first.