A reset should bring back default values.
It's the same for both outputs?
Maybe a bad battery, they last 5 years max, or failing power supply capacitors if the unit is very old.

Hard to diagnose without a oscilloscope, but could be some wrong resistor values. Some values need adjusting to match different transistor batches. Or maybe wrong output transformers.
Can you upload a closeup of the output stage where we can see the color rings of the resistors?
I really do not ...