If they're that old, you could have some caps going out on the amp board. Be my quick & cheapest guess...
Or actually first off, did you try cleaning the pot switch on the back on it yet? (If the pic I'm looking at is right) I've had 2 subs do that to me...but you usually get some scratchiness out it if it's that. But that looks like just a bass level so it's probably not that.
Good luck, see what everyone else thinks!
Pessimist: Oh, this can't get any worse!
Optimist: Yes, it can!