He encontrado esta pagina : http://rkerkhof.ruhosting.nl/Taas/Mods/Big%20Muff.htm
Y parece ser que esta es la modificacion que quiero, añadir un control de medios:
Mid control mod: This mod is beloved by both guitar players as by bassplayers to regain the mids in the sound, mids that are so needed to be heard in the mix. With the next control you can dial in anything from a (huge) mid cut to a mid boost. Note that by boosting the mids you'll lose the Muff's character; still, it is a very nice addition!
Replace C1 with a 22n cap
Replace R2 with a 22k to 25k potmeter (wired as a variable resistor) with a 1k resistor wired in series.
Tambien buscando por la red encontre una pagina que me hacia la modificacion: http://www.peptoneelectronics.com/products/Big-Muff-Fuzz-Mod%3A-We-Mod-Your-NYC-Muff.html pero claro, cuesta mucho dinero, prefiero hacermela yo. El objetivo sería este: http://www.youtube.com/watch?v=u53rLj8Epc4
Me resultaria costoso o complicado hacersela ? La verdad esque yo no entiendo mucho de circuitos, pero parece sencillo.