A pre-amp is only essential for a mic as far as I can tell.

You gigging yet Anna? I'd come and see ya.

Depends on the mixer. If you have a small mixer, it usually has a built-in pre-amp, so you should not need a pre-amp at all, even for a mic (mixers often have specific mic inputs). Want more bass in the mix? Surely that's what your mixer is for? I don't just mean the level, but modern digital/analog mixers often allow you to add effects too. Something like this:
http://www.m-audio.com/products/en_us/NRV10-main.html

..or just use an effects processor between the Bass and mixer? That will allow you to simulate a wide range of amp sounds making it sound a lot less clinical. Not as good as using a proper tube Amp, but still pretty amazing what some of these can do.

Alas nope not gigging yet due to a most unfortunate injury of the wrist which has rendered me unable. Soon though soon, and you'll be the first to hear of it. Thanks for the help.