I thought I'd use this, but I have new basses on the way and just can't take the time to have extra gear around. It's a great little unit though. I used it on my active bass as well, although I don't know if that's recommended.
Stellartone Tonestyler for Bass
The ToneStyler BASS™ is the new lowest-voiced version of the original ToneStyler®. This unique control shifts the pickup’s resonant frequency and adjusts the treble roll-off point in sixteen 0.44 octave steps. Works with all passive bass pickups - no batteries required! 100% pure analog tone – ZERO noise – aerospace grade construction and components. Maximum capacitance value: .1µF
The EQ range is extended two octaves lower, which provides additional resonance selections in the upper bass & low midrange harmonics. The BASS model is ideal for shaping & enhancing the pickup response in all passive bass guitars.
3/8" Medium Length, Split-shaft, Notched Rotation Potentiometer
$85 + whatever shipping is which should be a couple bucks since it's so small. Paypal is great.
Stellartone Tonestyler 
