I'll discuss preamp tubes this issue and output (power) tubes next Not in the least: when used to amplify electric guitars, tubes still simply sound better than anything else out there. (if not more) continue to use tube amps for both recording and touring, . Fractal Axe-FX II Preamp FX Processor Review. Choosing pre-amp tubes by gain factor. One simple tweak that is favored by guitar amp users is to plug in a pre-amp tube with a slightly different gain factor. A tube guitar preamp pedal with a huge range of tonal dynamics! The SS Guitar Preamp . (5 of 6 customers found this review helpful). Set my Marshall.