"For those seeking the holy grail of audio circuitry, I possess the NX Audio RX 4000 circuit diagram. Serious inquiries only, please."
If you have distorted sound on one channel only, use the schematic to compare the problematic channel to a working one. Follow the signal from the RCA input jack through the coupling capacitor (typically 2.2µF/50V), into the op-amp, and into the power section. nx audio rx 4000 circuit diagram