Pi vs BigMuff
the 'Pi' version has (3) options for adjusting the sound: 1. volume 2. distortion 3. tone* *and turning the 'tone' knob counter-clockwise will give you a more and more BASSier sound. this is primarily how the Pi differs and in my opinion is superior if you're a bass-player.. especially as the 'tone' knob is pretty easy to twist with the end of your shoe while performing, as needed..
i've had about 4 russian big muffs, and all have failed.. they don't take kindly to constant transportation + the 9volt battery plug can be accidentally torn if you're rushed and changing a battery at a gig, or in the back of a tour-van, or in a snow-storm in winnepeg, etc....
overall, the two pedals have a somewhat distinctly 'different' sound/fuzz/tone.. i kind of like them both seperately for what they do. i've yet to try two at the same time - maybe i will later today, hmmmm...