Billy's Electro Harmonix Big Muff π, with the SD-era settings scratched in

Billy uses an old Big Muff π (pi), although by now it most likely has been rewired (due to the fact that old Big Muffs seemed to screw-up inside - as in the wires would be degraded). He has a early 70's model, which is what he appears to use most of the time. He may use a reissued version at some shows, but this has not been confirmed.
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2 Big Muff πs are shown in a video of Billy's huge collection of guitar pedals called Stompland, including one with the knob settings used on Siamese Dream scratched in (volume, tone and sustain set to approximately 12:00, 1:00, and 5:00 (all the way up), respectively), also pictured here.

Billy wrote a big paragraph on the Big Muff, which was posted in Kerry's blog on 2009/10/27:

My first mental recognition of the 'power' of the Big Muff came one night when i just happened to drop by a Catherine band practice. Catherine was Kerry's band in the 90's, and we used to share the same rehearsal space (they were the only band we trusted not to steal our shit and vice-versa). So i walked in and i just heard this tremendous ROAR and i thought to myself 'how are they making that sound?'. Each of the 3 guitarists had Big Muffs and the sounds was very Sabbath-like, just throwing this tremendous low drone buzz into the room. I knew that the band Mudhoney used Big Muffs but honestly they weren't using them like Catherine did. So i owe a debt of gratitude to Catherine for showing me the purple fuzz light. We used to try to use the pedals live but it was impossible for there wasn't enough clarity. We did use other Electro-Harmonix pedals live, like the Electric Mistress. We only had one of these in the beginning and James and I used to take turns over who would use it on a given night. Since we were mostly playing clubs we were looking for anything to take your head off and separate the sound of the guitar from the incessant din. The Big Muff pictured is 'the' one from Siamese Dream, the fuzz that launched a 1,000 other dreams i suppose. Almost all the heavy tracks were recorded thru this, solos included. Many songs would have as many as 8 fuzz guitars going at once. It was very difficult to record the tracks with this sound, but once it went right it was BLAMMO, a huge, huge sound. The settings you see in the picture are the exact settings for if you look close enough you can still see the pencil marks on the volume and tone. I had all sorts of theories of how to best use this pedal, including the idea that it only sounded its 'best' when used with a battery (as opposed to a power supply). I'm not much for going backwards but it is tempting to crack out all the old gear sometime and give it one more go. Maybe somewhere in amongst the 44 songs of Teargaden by Kaleidyscope I can find one such moment to do so. The sound of the combo of the Bat Strat, the Big Muff, and the Soul head with Mars cabinet is still unmistakable.

One quick funny story: I used to shop at this music store in San Francisco, and in the early 90's they were selling these pedals back then for about 40 bucks because nobody wanted them. About a year after Siamese came out, '94 i guess, I went to the same store to see if they still had any. The guy behind the counter knew me from before, and told me, 'yeah, i got a couple.' I said 'ok, how much?' '200 bucks' he said casually (which at that time was A LOT for a vintage pedal. '200 bucks!!!! Why are they so much?? Are they rare or unusual?' He said, 'no, but ever since you talked about using them we've sold about 75 vintage muffs. Sorry man, they're all looking for your sound!'. What he didn't know was they were really looking for the Catherine sound.