there is a "Hum Balance" pot in the amp but that doesn't seem to be your issue..... After you make sure it's none of the tubes, the amp is biased properly, it's not a guitar cable, it's not your guitar, and it makes the same hum at someone else's house (it might be some VERY dirty power in your house but I doubt that too) then check your hum balancer. Here's a picture of it on the board. It's probably a pre-amp tube or guitar/cable though. GOOD LUCK!!!!!!!
-Alex