Consider tubes like the tires on a car. For best performance they need to be properly maintained: kept at the right pressure and replaced when worn. The right pressure is similar to the correct power tube biasing: too hot and the tubes will wear faster, too cool and the tubes may last longer but tone may suffer. Power tubes draw more current than pre-amplifier tubes so as a result wear faster and will need to be replaced more often. Think of replacing power tubes at a 1 to 3 year frequency, depending on how often you play, and possibly double that for pre-amplifier tubes.
Fat One amplifiers with adjustable bias are set to 60% power dissipation at the factory for great tone and long tube life.
