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.
