Alex,
Your guess is correct, the bias is way off on your opamp (the TL074). Each pin should be biased to around 4.5 volts (half the supply voltage), but for some reason it’s a at 1.5v. In these types of situations the opamp doesn’t usually go bad or break, the issue usually is that the circuit sending the bias voltage got messed up somehow. You need to check (re-flow) the solder joints on R9 and R10 (the junction that the 4.5v is comes from) and also C15 (also make sure this cap is installed in the correct orientation). Also check that C10, C11, and C16 are soldered correctly. It kind of sounds like something is partially shorted to ground…like the paper insulation under the pots might have been punctured by a lead and is touching the back of one of the pots. In a rare case, it is possible that the opamp went bad and needs to be replaced. If you can’t seem to get the bias voltage back up to 4.5V by doing the things i just suggested then we’ll have to assume that the problem is the opamp.
Good luck.
-Brach