Anyone tried out Kasha Overdrive Pedal? It's made for Guitar, but their big selling point to Bass players is how it acts as a "compressor" and evens out the notes...here's what they say:
Bass Guitar: The pedal brings advantage to a bass players rig. When using with an active bass, the pedal will make all notes even which will result in in an active bass sounding analog or passive. In addition, it will bring forward your notes with more articulation and dynamics. The first thing you will notice is that the notes will sound full and richer than you ever heard them. Then when playing up and down the fret board, you notes will be even without the peaks and valleys heard within the tone spectrum.
Here's Rudy Sarzo talking about the pedal: http://www.youtube.com/watch?v=PQ4PMb8eIIo