#1
WHAT THE HELL IS BIG MUFF? ive heard nuthin but great things about it and know that it exists in both circit and pedal form, but someone please explain!
#2
More or less the original muff. http://www.ehx.com/products/big-muff-pi

The circuit has been changed and tweaked hundreds of times, being made in tons of different sounding models. Aside from the original Muff that I linked. Some other examples of the Muff circuit include the Blackout Effectors Musket Fuzz, the Black Arts Toneworks Pharoah and the Skreddy Cognitive Dissonance.

EHX themselves have revised and changed the muff numerous times as well. It's a very common fuzz circuit, and for good reason
#3
What makes them different is the "Pi" part. It's a capacitor in conjunction with the clipping diodes that gives it more of a soft-knee clip and makes them sound less harsh than a normal fuzz.
Gilchrist custom
Yamaha SBG500
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Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
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Atomic Amplifire
Marshall 1960A
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My band