I've never used separate preamps, but I've heard people talking good about SansAmp and Radial preamps, but there are other brands as well. Try out a few different models and see which one you like the most.
I would not recommend the same preamp for both bass and guitar. I have a guitar preamp, a Hughes & Kettner Tubeman, and you can't get a decent bass sound with it, I've tried. Also, most effect pedals meant for guitars don't work well for bass. If you want to use the same for both, go for a bass preamp.
EDIT: Just make sure the preamp has a DI out. (I guess most preamp have one but with my limited experience with separate preamps I can't say that's always the case). You use the DI out to connect the pedal with the mixer.