Hi, you can monitor the error signal (so basically just the distortion part) on the monitor outputs using an oscilloscope. That should give you a clue to what is wrong. Post results.
That said, this thing has a linear power supply that has a design flaw. The electrolytic capacitors are right next to a big heat sink and prone to dry out. I would advise you to use a scope to check if there is 100Hz ripple on the power rails. Since these devices date back to the 90s, I wouldn't be surprised if those caps are dry.