How is it that different brands of tubes produce different (warmer, clearer, brighter) sound? Isn't it true that a tube does nothing but provide power to different parts of the amp? What are some of the better tubes out there? I am getting ready to take my Fender Supersonic to a local shop that specializes in tube electronics, most notably guitar amps. I don't want to get ripped off because I am dumb regarding tubes. How much should I pay for a decent tube(preamp and power)?