Hard to tell with this scenario. You will need a tube tester and test the tube for emissions and a life test to verify the tube operating within spec.
I'm not familiar with the amplifier but if you move the tube to the opposite channel and perform the same output test on the left/right channels to verify the problem moved with the tube. Otherwise the problem may be in the circuit for that tube location, resistor most likely but the circuit will need testing to verify which component is out of spec.
I'm not familiar with the amplifier but if you move the tube to the opposite channel and perform the same output test on the left/right channels to verify the problem moved with the tube. Otherwise the problem may be in the circuit for that tube location, resistor most likely but the circuit will need testing to verify which component is out of spec.