Interesting BMP knobs...

[petemoore]

http://spenceramps.com/mixmuff.html
  Interesting number of knobs and also the approach is interesting
  The standard transistor Big Muff pedal contains three gain stages: a clean boost, a first distortion stage (overdrive), and the final distortion stage -- which is all you normally hear. The Mix Muff provides mixing capability to select from and combine all 3 of these great sounds. The overdrive stage is also reversed in polarity (antiphase, or inverted), subtracting from the clean and distortion signals, clashing with them, so a fuzzier effect can also be generated.
   ...an NPN/PNP mix, maybe opamps using + input here..-input there ?
  Interesting two/fold tone !