One of the best preamps I've ever used was one that I had John East custom build for my P/MM bass project. It's three stacked knobs - Von/vol, B/T-B neck and B/T-B bridge with the neck stack being voiced for a precision bass and the bridge stack voiced for a Stingray bass. Each pickup has its own dedicated preamp and they tie together into a stereo output Jack. The preamp also has a three way selector (neck, both, bridge). This arrangement gives me the option to run one pickup (usually the neck) clean to an amp and the other pickup (usually the stingray pup) to effects and on to another amp or blended with the neck pup or possibly to a stage monitor. It works great. Not including a battery, it all fit on a standard jazz plate in a standard jazz route.
He uses mini board mounted stacked pots so they don't take up much room. Here's a pic...