Thanks, all. Sensible advice (I assume that Rothmanbrad was more or less joking, but that his point was "pick an answer in the ballpark and don't obsess about it" - entirely reasonable, imho - then again, the Baron uses 12 output tubes, so replacement with nos is not exactly a trivial expense - so far, I prefer Siemans EL34's). I forgot that Mr. Smith addressed the issue in the manual; indeed, who would know better? I rebiased (easy on a Baron) and will forget about it for another year. I do keep spare sets, but they all sound a little different.
how long should you use tubes in an amp?
As tubes age and they lose power, is the degradation linear? My Mesa Baron uses 6 tubes per channel, and the speakers are efficient so I never use more than 10% of the output power, even on peaks, even in all-triode mode - and most of the time, it's far less. The Baron has meters, so it's easy to see this (in fact, it has switches to reduce the meter range by a factor of ten - they would never budge, otherwise). So if no more than a fraction of the full output power of the tubes is required, why not use them well past their "normal" lifespan - and just turn up the volume? Do the tubes lose some dynamic or other capability, or is it just a "smooth" decline in power?