"The "Guv'nor" pedal, officially Marshall The Guv'nor, is based on replicating the sound of a cranked Marshall tube amplifier stack (like a JCM800) in a compact pedal"
You know you can use an amp block as a pedal right?
Add a 2nd amp block before the one that you use. Select a jcm 800 based amp, then turn off power amp modelling: "Go to the Power Supply page in the Amp block and set Supply Sag to 0. (Cliff)"