well, in a typical amp head,the pre amplifies your signal to line level..then slams it into the power amp section-a mesa boogie head ,for example,has pre amp tubes that help create that rich saturation/sustaining sound-this signal created by the pre is sent into decibel heaven by hitting the power amp(power tubes..transformers,etc)..the signal now can kill animals...tube amps create different tones based on the tubes/transformers/etc-a boogie has the trademark preamp gain sound,while an older marshall is known for its power amp obliteration.. :twisted: