The Big Fuzz is a muff circuit, which has been critically re-tuned to improve the tone, playability, and versatility of the pedal.
The MIDS control knob sweeps from the classic scoop, through to a more focused modern tone, with everything nice in the middle.
The BMP/STB switch toggles between a modified muff setup, to working alike a 1973 Colorsound Supa Tonebender (with the variable mid knob, of course).
This fuzz uses gain-selected transistors, and black glass silicon diodes. Due to signal processing, this effect is not particularly problematic with input or signal chain positioning.