That's an awesome head. The bias adjustment, I think, runs the tubes hotter if you turn it clockwise, colder if you go CCW. Pre-amp tubes usually don't need to be replaced that often, and I wouldn't mess with those if you're happy with the overall sound. Power Tubes, on the other hand need to be replaced at least twice a year, more if you use it all the time. I wouldn't change the tubes anyway unless you noticed a difference in the sound. As for experimentinfg with that bias knob, go for it! Just change it a little at a time, because Peavey tends to bias their amps a little colder than what I like, producing crossover distortion, which is an underlying buzzy-ness. This does prolong the tube's lives a little bit, but it doesn't sound as nice as when you run em hot. Hope this helps.