Time to get something new. Why fool with a 30 year old preamp that can’t be serviced?
Try contacting Joseph Chow of Audio Horizons. He also has a repair/modification company called Component plus, he can tell you whether it's worth fixing or not.
I contacted Joseph and he responded quickly. Not necessarily offering repair service, but he gave me advice to deal with the static on the volume knob: using contact spray inside the volume pot.
I removed the cover and unfortunately found that the volume pot on this unit is sealed in a square housing, and is rather difficult to reach. I did manage to get the contact cleaner straw inside a little hole in the housing and spray a few shots. It apparently was enough, as the static is gone now.
Haven't asked him yet about the sound processing issue - but I imagine that will be hard to fix as it's so intermittent. The digital inputs have never worked and obviously don't hinder the use of the machine.