[RodB]

Depends on what test equipment you have. You will want to narrow it down to one of the audio stages. If you have a scope and audio generator feed audio at the top of the volume control then look at the signal as it passes through each section, looking for the distortion. If you don't have test equipment, use headphones or small speaker with a cap in series and listen for the sound as it travels through. I would look at a couple points in particular. One is the volume control wiper. Maybe some more cleaner there. The second is what looks like a negative feedback circuit made of C16 and R12. Open the circuit feeding to the volume control and see if the distortion goes away. If you haven't found it, temporarily replace C14. These do fail occasionally. I don't see a cathode bypass cap on the output tube, try a 10 - 20mfd 25 volt cap. I assume the 2 tubes were verified. Check bias voltages.