Sometimes it's obvious when a preamp tube goes bad (it becomes noisy, microphonic, etc). It's not as obvious to me when (or if) a preamp tube has simply lost its tone without having any other specific problems develop. I say 'if' because I was reading the manual for the Mesa Studio .22 I just got, and the manual recommends never replacing the preamp tubes unless a specific problem develops or the user wants to experiment with the sound. Now, this is an old manual--it's the old typewritten style, and the amp design dates from 1985, so some realities as far as tubes go are bound to have changed since then. However, I've only ever noticed the tiniest of changes when replacing one set of current-production preamp tubes of unknown age in an amp with another new set of current-production tubes. Am I wasting money by doing this as a matter of course? I just ordered a retube for my Studio .22, and I'd feel pretty silly if I dropped a bunch of cash on preamp tubes when only the power tube change is going to have a large effect. Plus, I have a whole bag full of various 12AX7s pulled from various amps, and I'd like to know if it's just as well to use them.