There are several ways to tackle this....
1) Turn the volume knob of your bass down.
2) Lower the pickup on your bass.
3) There are 3 potential gain/volume settings on the B2--The global volume, the ZNR module volume/gain, and the preamp module's gain. The last two are where to adjust for individual patches--and are likely where your problem is.
Unless your B2 happens to be defective (which I doubt), the problem should be easy enough to resolve. I have a couple of very high output basses, and I can dime the volume on both without distorting the B2.