Have you tried different strings? Sometimes strings can make a really big difference on acoustic guitars and basses.
An outboard preamp is not going to do anything that the preamp which is built in to your amp won't do. Frankly, IME most piezos suck. Look into replacing the piezo with a K+K transducer pickup. They are somewhat easy to install yourself or if you're nervous have your local guitar shop do it.
I was unhappy with the piezo in my Seagull guitar, and I replaced it with a K+K and the difference was like night and day. A friend of mine replaced the expensive and exotic pickup/microphone setup in his Taylor guitar with a K+K and the guitar sounds 100 times better than it ever did with the convoluted mic/piezo mixer thing. The K+K will do more toward giving you a nice natural acoustic tone than a bank of preamps.
If my Thunderchief had conventional bracing and sound hole I would have installed one in there as well; I am not very happy with the Fishman piezo in the Tacoma.
FWIW my brother Peter's Wechter guitar sounded very good with the on-board piezo pickup, he used a very nice Fender Acoustisonic amp.
I want to see pictures of your bass. Is it solid walnut back and sides like my brother's guitar? Goregeous construction!
