I Play mainly rock and power jazz and want some opinions on preamps. I have a DLX Jazz and love the onboard preamp, but want to get something for my Passive and future basses.

Which is better, and why. An onboard, like Aguilar, or in-line like sansamp?

I like the convinience of the onboard preamp but if you need a preamp for more than one bass or don't want to modify a vintage or collectable bass then inline is the answer. Also, most onboard preamps are strictly for boosted output and EQ. If you need effects, overdrive or an included DI you are once again going to have to go with an inline unit.