One of the easiest ways to isolate an issue with a low or distorted signal, is to swap the two DOA op-amps with known working op-amps if you have some. Any 2520 pinout op-amp should work. This will isolate whether your issue is an op-amp issue, or an issue with the circuitry on the main PCB.
If your unit works fine with different op-amps, swap the original op-amps in one at a time to isolate the specific problem op-amp.