This is an interesting thread.
Whenever I meet a customer who is unhappy with the amplified voice of their instrument, I tend to suggest a pickup swap as opposed to a preamp installation. Dfwkeys, if you can tweak the eq settings on your amp to get a sound you like then perhaps a preamp is what you need, but a change in pickups will directly change the amplified voice of the bass.
Many stock Fender pickups (from MIM to FCS) are pretty low in the pecking order for reasons involving profit margins. There are several brands to check out for replacements like Fralin, Nordstrand, and Lollar just to name a few.
Ultimately, the tonal response and feel of a bass begins with the resonance of the wood and how it relates to what you want to hear. From there, the pickups are the next step, followed by a preamp if you want more variation in shaping your sound. Incidentally, brands and gauges of strings can have a huge effect on changing the sound of the bass to suit your tastes.
That's generally how I approach this sort of thing. Just my two cents.