[standard copy and paste Big Muff explination]
I've answered this many times and somewhere is the GREATEST Big Muff breakdown ever, but I can't find it, so I'll try again:
There are four variations:
*Sovtek Green (Huge, discontinued)
*Sovtek Black (Smaller, current)
*NYC Reissue (Reissue of the original)
*NYC Original (The original EHX Fuzz)
I can't vouch for the sound differences between the original NYC Big Muff and the reissue, but my weapon of choice is a modded NYC RI BMP, the blue light special. The Sovteks are obviously imported from Russia, but that doesn't mean they're bad; they're different. The Ruskies tend to be a little colder sounding as they're made with different parts, the interior work isn't as clean and they don't come with a power adapter, but everything can be fixed.
The American's tend to be a little warmer sounding as they're made with quality parts. The case is a tad more flexible under weight, but they hold up for years. I recently came across a 1977 BMP that was still rockin' hard after 26 years. Though it was after my Blue Light conversion so I couldn't A/B the tones.
The difference, as far as I know between the Green and Black Sovteks is the case. The Green one was massive and the black one is smaller, both are sturdy and contain the same circuit.
But when looking for a Muff, they tend to vary a bit in tone from model to model, some warmer, some crispier than others. I suggest you try each before buying.
[/standard copy and paste Big Muff explination]
Try a search next time.