Good work! One trick would be to short the inputs and see if the 60 Hz goes away. If the hum follows the volume pot, then it's in the front end somewhere, maybe in the inputs. Be sure it's buttoned up (all covers in place) when you check it. Make sure that the shield/braid on the input coax coming from the input jacks is not grounded at both ends. It should be grounded at the end at the input jack itself. Sometimes you'll get a ground loop and 60 Hz hum if these shields are grounded at both ends.
An amp of this type is by nature stable and is going to be hard to drive into distortion or the kind of guitar sound you may be looking for, but as a mono block, not bad (as you say).
Electrolytic cap problems will cause 120 Hz buzz, instead of 60 Hz hum. If you had one of these backwards, it wouldn't last long before it heated up and failed. For film couplers, there really is no 'backwards' way to connect them.
Good luck with your efforts.
Dave
