There's a few big muff sounds on the b3... and compared to most other multis I've tried they're actually decent... nothing will ever beat a dedicated, analog stomp box distortion. I have a bass big muff that I use with my b3 and it has a lot more character than the modeled version, less sterile and more gainy. There's many other great muff clones out there that simply stomp either of these though (wren & cuff models, fuzzrocious grey stache, diy pedals, etc).
In summary, the modeled muff sounds are good, not great. Spend a little extra and get the real thing to go along with your nifty multi effect unit.