Re: Electro harmonix Muff fuzz
for starters:
You can replace the transistors with higher / lower hfe values to increase / decrease the gain of each stage.
Change the clipping diodes to germanium or even LEDs to change the clipping point.
Add or remove a clipping diode for asymmetrical clipping.
Replace all the ceramic and electrolytic capacitors with poly / metal / mica film capacitors to reduce the harshness.
Change the values in the tone stack to shift the range of the tone control, or replace it entirely with a baxandall type control using a dual concentric pot.
Install a true-bypass switch (if your pedal is an older / Russian model)
Change the color of the LED (serves no purpose – looks cool?)
Add a power adapter socket if your pedal doesn’t have one