Here's a cool DIY design of a Green (Mutamp) amp emulator. Four JFET stages with a 2-band EQ and a Boost control. Should be a nice easy fit in a 125B with the board mounted pots. Here's the schematic for reference. Sound demo here.
This is a library of perfboard and single-sided PCB effect layouts for guitar and bass. I'm not an electrical engineer by any stretch of the imagination, just a DIY'er who likes drawing layouts. It is meant for the hobbyist (so commercial use of any of these layout is not allowed without permission) and as a way to give back to the online DIY community.