I don't know about the Ref 110, but I think it is probably similar to the VT100, since they both use a quad of 6550s per channel, biased similarly. My VT100 Mk II went close to 2000 hrs on the output tubes, which for me was 6 years (I guess I don't listen that much). I replaced the output tubes only. However, I did adjust both the DC balance and bias when I installed the new tubes. All the small tubes are still original. Next time the outputs need changing, I'll change them all. Probably in about 5 years.
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