I eventually called and asked for a date certain about when I would get an estimate. Note that they did not call me. The owner said that he does have the estimate and that it's $600, far above what I think is reasonable. He gave no details at all over the phone about what would be needed and said my choice is to pick it up for $100 or go ahead for $600. Somehow I doubt he'd ever fix it for $600.
For now, I'm considering just abandoning it in the shop and buying another amp.
For now, I'm considering just abandoning it in the shop and buying another amp.