I've vacuumed the inside, found a dead spider and apparent mouse droppings. Pulled the face (all the knobs were a bear to pull off).
The unit seems to sound fine through headphones, although there is a base 60Hz hum which does not increase with the volume. I'm waiting for my Caig chemicals to arrive, then I'll clean it up proper and determine the cause of the hum. Hi-Fi Engine had the service manual, so I'm good to go.