Shortly before it was released, I shot an email about that very detail. Here is a bit from their response:

The Bass Big Muff is based on a really good sounding current production Russian Big Muff (we don’t have any of the green ones unfortunately) with some changes to the Tone knob to make it sound better with bass. And then we added a switch to give a bass boost or mix in your dry signal or no change at all.

If I weren’t already quite pleased with my Black Russian (named, get this, Black Russian), I’d get one.