My Muff was an NYC model I bought for dirt cheap. I then modded it to more or less a green russian one.
The mods I have done are some of the most frequent I think. 2 switches are for removing the diodes from the circuit (one switch for each diode pair), another switch is to have a mid bump, and the last one is to choose between two resistors somewhere in the circuit which changes the character of the low end. All mods are very usable in my opinion, both on bass and guitar.
Removing the diodes is such a cool trick: basicly you get a Colorsound Supa Tonebender, with the result of more volume (it really is loud), less compression and gain, thus a more open overdrive-like sound.