If you're feeling ok to experiment, you can try the advice given by the Fishman website for dealing with this type of problem installing their piezo pickups.
They recommend placing paper shims between the underside of the bridge and the piezo pickup itself, strategically placing them under the quiet strings, thereby creating better contact to even out the string volume. They say you may have to fold thin bits of paper in places to create the appropriate thickness shim.
Note this solution is based on the assumption that there are uneven gaps between the underside of the bridge and the piezo pickup, leading to uneven string volume. It's also assuming the piezo contacts are working up to spec.
BTW, are you using factory spec. string gauges on this bass ?
Hope you get it sorted.