Hi Mattinthebox, welcome to forum,
Have you verified that the pin on the yellow wire has become detached from the mosfet. I can't quite work it out from the picture as to whether those FETs are in sockets or just mounted to a piece of chassis. If they are in sockets then it would be good idea to investigate the pins and the fit. Maybe the socket is the problem, or part of the problem and not the FET. Might be wise to dismantle ad check the fit.
If it was a bad connection to the FET that would tie in with a LOUD pop.
The toast marks on the board are not too bad. You should measure those components and if they check out ok, then resolder. Best approach is to desolder the part, clean the board with solvent like IPA (alchohol) and toothbrush to ensure good joint. If hard to get a good joint, then you can wirewoll the resistor legs to remove any oxidisation.
You should also google dim light bulb tester and consider making one up. It will help prevent damage to the amp in case anything goes wrong with the disassembly et