Based on the percussive transients idiomatic to the nature of a banjo, coupled with running the pickup signal through a preamp prior to the Spider Jam, all these components factor into "clipping" the input of the Spider Jam amplifier.

Does your Fishman preamp not have the ability to cut the output volume? Or is it fixed (non-controllable)?

Maybe something like this: http://whirlwindusa.com/catalog/black-boxes-effects-and-dis/direct-boxes/imp2 will help by converting your preamp's output into a mic-level XLR signal. I'm not 100% sure on this recommendation as I have never encountered a banjo player running a transducer/preamp into the Spider Jam before.