One channel of AMP has distortion. Where do I start?

They are MPC 725 Low Ohm Power Resistors.

https://www.electro-tech-online.com/custompdfs/2009/04/mpc75.pdf

as far as the bias transistors (Q554 and Q555) go, the only distortion they would cause would be distortion only at low signal levels that would disappear at higher signal levels. this would be caused by the bias being completely shut down.