What does the FX Mix/FX Loop Mix Knob do?
If your Marshall is equipped with a front panel FX Mix/FX Loop Mix knob (Fig. 4.1 & 4.2), this indicates that the amp has a Parallel or Series/Parallel effects loop. This means that you can adjust the amount of effect that is heard via this knob. If your amp does not have this knob, you have a Series Loop, in this instance you can adjust the effect level via the effects unit itself.