[Jeffrey Brown guitarfool]

Just my 2 cents: I would swap out the opamp IC – If they already thought of that, then never mind. The distortion is a clue. opamps that have had static discharge damage may appear to be perfectly fine when new. The damage often doesn’t rear its ugly head until years later, when the damage actually causes failure. It’s like a time bomb. I’ve seen it several times.

Other than that, cold or bad solder joints on the board can also fail after a long time. Remelting them usually helps.