I like my external preamp (Sadowsky) as it offers frequencies my amp doesn't and, as a side benefit, it buffers my signal. By not having it in my bass, I only need one preamp, not one for each bass. The downside is having to have my pedalboard available, but as I also use a compressor, chorus, octave and pedal tuner, I'd probably be taking it anyway. If I only had one bass, I'd probably fit an Aguilar OPB1 as that does the same job as the pedal and free up some room on the board.
EDIT: Holy thread resurrection, Batman!