There are three things that make amp: Preamp, power amp and speaker. From input your signal goes to the preamp section. You can configure your sound here (gain, treble, midle, bass, etc.). Than, your sound goes to power amp section where your signal from preamp gets "louder". After that the whole thing goes to the speaker(s) and that's it!!! Preamp (like Marshall JMP1) is just part of a ordinary amp (the part with the knobs)... Preamps are good for recording and sometimes even for live situations... I'm owner of BOSS FX-700 preamp procesor (it has integrated multi FX)... I use it alot for home recording and it's great... Live situations... Only as a part of my FX send/return chain of my trusty Marshall... I hope that I answered your Q.
