Moscode 300 and hum

I have a 300 which hasn't been in service for a few monthis while I moved my equipment. When I hooked it up there was a distinct 60hz from the speakers. I figured it was a ground loop and tried disconnecting all the cables and got some decreased hum but not enough to eliminate it. It was quiet before. After a few days the hum is almost gone, without changing anything else. Would the disuse for a few months affect the filter caps and cause the hum? Did they reform? Is this some indication that the caps need to be replaced??