OK, first look at the speaker cone. When you turn the amp on, does the speaker cone move one direction and STAY THERE? Or does it maybe bump at first but settle donw to about where it was before?
MY experience tells me the most likely problem is the two large main filter caps sticking up from the center of the main board need to be resoldererd.
Ball up your fist and whack the top of the amp. Does that momentarily stop the hum?