I'm having a hard time accepting the assessment. The amp chassis looks, cosmetically, to be in great shape which leads me to believe that it's been in agreeable location, climate wise, as opposed to say a damp basement or out in a garage. I also find it hard to believe that ALL of the tubes are weak. I'm not trying to disrepect the technician but the 4 figure estimate to restore it sounds a bit too much.
Is there any possibility you can remove the bottom panel and take a clear close up picture of the internals? if so hopefully some of the resident tube gurus might be able to spot something.
One last thing, hold on to those tube regardless. I've seen tubes measure "weak" that on another tester tested fine.