IMO, they do sound best when played through clean tube amps with lots of head room and little breakup. I have never had great results using them with amps that already have dirt, though some people do. The dual clippers in the Muff circuit create a very scooped distortion on their own, so throwing that signal again into another circuit making more dirt out of dirt can sound bad. If the amp is too distorted the lows can bottom out and you can lose all your clarity.

Billy’s amp was a Marshall JCM 800. Not exactly a clean amp, but based on the sound, pix of his Muff, and the pots, he was likely using an op-amp Big Muff, not a transistor Muff like the LBM. Op-amps react slightly different to dirt than the transistor Muffs do.

I don’t know if I would call using those China tubes much of a mod, but there would be more headroom. I have played a stock EL34 JCM 800 with my V4 op-amp Muff and I can say that Marshal tone I hear on SD really comes through with that particular amp slightly dirty. I get great results with my very clean Reeves Custom 50 too, another EL34 amp.

I’m sure there are mods that would make a Muff work better with a dirty amp, but it is a gamble whether or not you would get the Corgan tone you are after with it. I would not even want to attempt messing with those mircro SMD components on that LBM board either.