| I use a Sadowsky outboard preamp with my Jazz bass. Kind of hard to describe what it does, though. It just makes my Jazz sound better, especially with a band. More focused, cuts through better.
Basses with onboard preamps tend to have the ability to sound brighter and/or bassier. A good onboard preamp wont affect your tone much when set flat, IMHO. This is somewhat rare, so I generall stick with passive basses. I've heard good preamps that were made by Sadowsky, Aguilar, Bartolini, and Lakland. I'm sure there's others.
Some preamps are designed to color your tone in a very specific way. The Sansamp Bass Drivers, for example, generally give you a more "tube" sound.
Past your amp, preamps are optional. Just a tool to help you get "your" sound. | 
