tone is totally subjective. that being said... i totally disagree with gear junkie.
all the usa muffs i've used are cleaner(by a hair), slightly more transparent, and less saturated than the russian big muff pi's. the US ones are the ones that all the bassist's I'VE seen like to use. the russian ones drop a lot of the low end of the original signal because it's fuzz is built so much on the upper end harmonics, when you use it to distort bass, it just disappears in the mix, a live mix especially. the us ones are a big smoother however, the russian ones will get a little buzzsawy sometimes. and if you crank either of them, it will feel like your ribcage is gonna cave in.
so i have just had totally different experiences than gearjunkie. as with any pedal, do your best to try one out, see which one you like, then buy.