I've only spent a short amount of time with the two, but to my ears the Iron Bell has a bit more gain available or at least doesn't do the "low gain" sounds as well. It has a bit more hair around the edges than what I'm hearing in the Colossus, but maybe that's what Brad is describing on his site where it says "Unlike the Colossus Fuzz which is based on the Sovtek/Civil War era Muff, the Iron Bell is voiced to be very open and rich, with an almost 3D like harmonic quality. "