Is the sound coming out of both channels or just one side?
If it is just one side, try swapping the left and right output tubes and see if it goes to the other channel. If so, it is a bad output tube. If you don't know how many hours are on the output tubes, the smart bet is to replace them to be safe.
If it is just one side, try swapping the left and right output tubes and see if it goes to the other channel. If so, it is a bad output tube. If you don't know how many hours are on the output tubes, the smart bet is to replace them to be safe.