I'm slightly hesitant to tell you this, because a) it's not EXACTLY what you're asking for and b) it's one of MY secret weapons, but...
Earthquaker Devices Bit Commander.
They bill it as a monophonic synth, and it IS that, but here's the thing: it's a crazy good fuzz. It takes your signal and splits it into four octaves: the orginal, one up and two down. Each of those is then run into a fuzz, some gated/some not, some softer/some more aggressive. You've got a knob to control how much of each octave is mixed in, plus another to control the overall tonal quality. The end result, for both guitar and bass, is a HUGE and hugely detailed fuzz orchestra.