I would bring the amp to you local ARC dealer. Even if you did not buy the amps from him he is still your dealer. He probably has resistors and other small parts on hand. He may fix the problem for free just to show some good will. Hoping you will buy from him in the future.
I had a similar problem and needed to replace a couple of resistors when I blew a tube. I called ARC, as I have done so many times in the past 30 years. They told me I needed to send them an email. I remember when they wouldn’t use email. After over a week of waiting they replied to my email. "You need to go through your dealer for parts". That was it! No help at all. The service department is not what it once was under Leonard or even Kalvin. I remember a time when Leonard would toss a couple resistors in an envelope and send them to you no charge.
My dealer had the resistors and replaced the parts while I went and had lunch. So it all worked out in the end.