Hello my friend, I have a Marshall MG 100DFX ("Almost" the same). I use this:
4 Cable Method
Mode - Stack Power Amp
1/4 Out - AMP
Stomp/Line - Set to Stomp
Input - NORMAL
The loop is after the "Mixer" and after the "AMP". Like you, it's preamp on preamp but sounds good. Also, in the mixer, I increased +3db on each path, you can check this video that for me is the best out there