Sounds like you damaged some transistors somehow (short the amp, over drive it or something). You should also check your speakers carefully or whatever was connected to the amp when you suspect it may have been damaged.
In Class AB designs they will run Class A at low levels and that seems to be where your problem lies since the problems are mitigated at higher outputs where other transistors are conducting (Class B amp section)
Note that Class A sections get hotter when they do nothing and cool down when they are worked harder - and this may be a related aspect of your probelm (why does it sound ok when tuned up and then fail when at low levels or off).
