Audio Research Reference 110

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.

Fred, I'm a REF 110 owner. Tube life will depend on how hard you drive the amp, quality of the AC power, etc. Audio Research suggests that 2000 hours is the time to start thinking about changing out the 6550C output tubes. There is a total hours counter on a LCD display on the inside, front, right hand side of the amp. I have had my amp for 18 months now and have about 900 hours. All is well.

I have had my Ref 110 for about 5 months, and have about 550 hours on the tubes. Over the weekend, I decided I should check the bias, which is VERY easy to do. It was still very close to original spec, at 65 mv DC. I only moved adjusted one, and that was off at 61 mv. Very impressive performance. That is why you spend extra and buy the ARC tested and burned in tubes...
Yes, at 2000 hours it is probably time to change the tubes.