As far as preamps... you really cant go wrong with any well known brands as they will all likely be worthwhile upgrades for your current bass however if you're looking to enhance the sound of your bass, the best place to look would be the pickups. The main objective of a good preamp should be to make the tone you have more flexible.
Back to preamps, my personal choice would Bartolini. They're relatively cheap, pre-wired, offer great tonal elbowroom, and sound clean.
Preamps aren't that difficult to install if you've had a reasonable amount of experience with electronics and a good soldering technique. I've had both (not an expert, or even a hobbyist, I just have done a few things here any there) and it took me like 3 hours to get my first preamp up and running. It worked perfect for two weeks then one day, right before a gig, I wasn't getting any sound. Took it to a tech and it turns out I didn't do awesome a job as I thought, lol. Moral of the story? If you don't think you can do it, you definitely can't and if you do think you can do it, something still might go wrong, so be careful
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I know nothing of preamps can someone help me? 
