ARC has to be able to warrant the repair. That means no tinkering or "close enough" parts. However that does not mean that the amp cannot be repaired. I would ask ARC for a detailed explanation of what they found and what would be needed to fix based on their findings. That would be your starting point toward finding a potential non ARC repair.
ARC M300 MKII call for help
Help! I have a pair of Audio Research M300 MKII mono blocks from yesteryear, and have enjoyed them for many years. One of them has a short in the panel behind the tubes. The guys down at the factory just got back to me, saying that they can no longer repair this model, due to age and lack of parts. Can anyone out there steer me to someone who might be able to repair it? I can send pictures of the shorted out area. Would hate to retire these beautiful behemoths.