Roger Modjeski, Isn't he the guy that sells tubes as well as some fine amps.
Interestingly, Randall Smith of Mesa Engineering who designed the Baron sez on page 18 of the Barons manual (I happen to own a Baron by the way) "Based on our experience with guitar amplifiers, you can expect 2 to 4 years of service before noticing any performance degredation. If none is noticed, and the Baron still sounds great, you may keep right on enjoying the original set with no worries. Often tubes will are have a warmer, mellower sound than fresh ones needing to break in. Should you think its time to replace........"
Randall seemed far more willing to be subjective in guaging the need for tube replacement.
Lets see, if I wanted advise about tube life in a Baron amp, who would I talk to. Hummmm, tough question!