Is it a 1/4" jack or a recessed pot? They look similar from the outside. If it is a recessed pot, then it could be a bias adjustment pot.
Other than that, maybe they added a line out and stuck the pot there even though it would be more appropriate in a different location.
Why not open the amp up and see what is really there and where it connects to? Then it would be easier to tell where it is going....
Greg