JL Audio RBC-1
Crutchfield Customer Review (What's this?)
I like this bass control knob. It helps a good deal when moving between different types of music. I installed mine in the front of my center console, within easy reach. As others have mentioned, this item should be a part of the package with JL's powered subs.
Pros: Works as advertised
Cons: Might want to consider including this with sub purchase