You can narrow this down to the AMP section by removing the shorting plugs (pre-amp out to AMP in metal plugs) and replacing them with an RCA cable.

Let assume that the left channel is not working. Using a standard RCA to RCA cable, connect the left pre-amp out to the right AMP-in. If the right channel has sound as expected, then the pre-amp section is OK and the problem is with the amp section.

With the power off using a meter, A/B the left and right channels. Measurements of the corresponding components should measure about the same.
If you have access without the possibility of shorting components, use a DC meter and measure some voltage points. Again, the two channels should measure the same.

With the symptom of slight sound in the output, I would suspect that a voltage is missing. You may have an open resistor that feeds the output section.