My brother-in-law has a couple of original Big Muffs. I took one of my Muff’n builds over to compare and the original Muffs have a huge amount of volume when compared to the Muff’n (ridiculously louder…). I know the Big Muff circuit is no secret so I have to assume the GPCB boards follow the original circuit with maybe the exception of the diode switch. The question is what make the original Big Muffs soooo unuseably loud?
I have not experienced that difference from Muffs made since the 90’s but that is about it. The oldest one I have is a Russian. You could easily make your Muff’n louder. Perhaps the answer to your question though is here?