Bob @ DST-Engineering is a well regarded amp builder/tech, he is north of Boston. I know quite a few people that have had work done by Az-Tech in Watertown.
You could email Alan Philips at CarolAnn....has worked on tons and tons of Marshalls.
He is crazy busy, but knows Marshalls better than a just about anyone.
Rick
AZ Tek in Watertown on N. Beacon St. Quality work, good turnaround time. I have been there several times and the work is always done right.
Tom (as I recall) is a true tech. He's not interested in NOS tubes or some mythical tone quest. But he knows circuits and how to make them right. If you go there, you'll see that a lot of folks agree.