The last McIntosh amp I worked on showed this issue on the preamplifier PC board. The traces are wrinkled and blistered. I've seen this before on other first-generation solid-state amplifiers. I presumed it was thermal distress from a de-soldering heat gun but was unable to replicate it using another board. I'm thinking it's a Quality problem with the board. This particular board came from an MC2505.
It works fine, the traces are still conducting but I've replaced it regardless with a new board.
distressed_board_2.jpg
distressed_board_1.jpg