buzz-hum from ml-2 amps

i just got a pair of ml-2's from a shop, needing a few minor repairs. when they are turned on there is a "buzz-hum" coming out both left and right speakers, from each amp. (quad 57's, stacked) its audible across the room. i have floated all the grounds to no avail. this is not a problem with another amp. the shop said they are perfectly quiet. any suggestions?