Without further input, I'd say you might want to look into the Tech 21 VT Bass pedal, especially for nailing those retro tube amp sounds. For even bigger distortion, the Darkglass B3K is getting a lot of attention. If you want something with a DI attached so you can send an ampy sound to the FOH, look into the SansAmp Bass Driver DI, the Hartke Bass Attack, The MXR M80 Bass DI+, the Aguilar Tone Hammer and the Darkglass B7K.
But all these units are really different from one another. Everything depends on the kind of distortion you're looking for.
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Ibanez Soundgear SR505, DR Hi-Beams
MXR M87 Bass Compressor & M80 Bass DI+
Source Audio Soundblox Pro Multiwave Bass Distortion & Envelope Filter
Gallien-Krueger MB500, Neo 212-II