Audio amp distortion
Check the negative supply rail to the channel, the bias level to the output transistors and the transistors themselves.
If you follow the signal from the input with a scope, you should come to a point where it goes bad, then you narrow down your suspect components.  

Also, tapping gently with a plastic implement around the board whist observing the wave can provide some useful information sometimes (connectors/wiring included in case of bad contacts)