Greg Christensen was the responding repair team tech.
Greg is the real deal, so I'm sorry to hear ARC can't help you with the M300s.
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.