Yeah, Carson was very helpful diagnosing the problem. He said that the issue is probably the output transistor for the the right Main and B outputs, so he has sent me some spare transistors free of charge! (They haven't arrived yet - I'm in Russia, so it'll take weeks for the package to go through custims, etc.) I have to say that I couldn't be happier with the customer service. Just wonderful!
Even better news is that the right outs have started working again! Having said that, I feel reassured that I've got some new transistors coming, because there's no guarantee that the problem won't happen again.