Bifwynne, Do you check the bias of your tubes regularly and replace them after about 2500 hours or so? The reason I ask is that seems awfully frequent to have problems.
I have owned ARC gear since the late 1980s. Everything from a D70 to my current Ref 110. In that time I have had a tube take out a resistor twice. And then once in the early 1990s I had to send my Sp9 to ARC because it would not come out of mute. That is it, no other problems. But I check my bias about once a month or so to make sure everything is fine. Tubes can drift, especially when the tubes are new.
Having a problem twice a year seems way to frequent.