The answer to this question has everything to do with your needs as a player.
First off, no amount of pedals is going to completely cover up the flaws of a sub par amplifier.
Beyond that, we're back to my initial comment and that leads to a number of questions that have already been posed in this thread:
- Wanna go ampless? If yes, then you need a preamp.
- Want a different flavor for your sound? If yes, preamps cover a lot of ground these days and it's much cheaper to go with a stomp box preamp than trying to track down and pay for vintage equipment (e.g., B-15, SVT, Fender bassman, etc.). Off hand, I'll say that Sushi Box FX and Broughton Audio offer some awesome emulations of classic gear in a stomp box format.