Sometimes the capacitors they use in these circuits will change value and will have no indication by just looking at them. I have changed many of these capacitors. The worst ones are the coupling caps between the finals and the output network, generally 500pf to 1000pf. Sometimes the ones on top of the cores are the culprit. Remove them and check them, If they are bad, I replace them with a good quality silver mica. Matching Mosfets are a good idea but not as critical as with bipolar transistors.