So I've been messing around a fair bit with magnets, pickup coils, and wacky wiring. I've come up with a crude reverb device that plays off of vibrations from the cab. This is very similar to the natural reverb you get when playing at very high volumes close to the speakers. The basis for this device is a pickup coil placed on top of the speaker cab with a separate magnet also on top of the cab but a certain distance away from the pickup coil. The vibration from the cab is transferred to pickup and the magnet nearby induces a current. You want the magnet rigidly fixed to the cabinet but just having it stuck on some piece of metal is good enough. You want to wire this pickup in parallel with your bass. Easiest way to do this is with a quarter inch instrument cable y connector. Now this is going create a gnarly feedback loop that's driven by the acoustical tuning of your cabinet. The effect can be moderate or extreme depending on how close you place the pick up to the magnet. It can be anything from a mild lightly sustaining reverb to a full-blown feedback loop that makes itself louder and louder and louder. You'd better have robust protection circuit of you try this and things get out of hand. There are a couple ways you can affect the behavior of this feedback loop. Obviously changing the distance between the pickup and the magnet will have a strong effect on the overall output. Additionally because the extra pickup coil load is wired in series with the bass any changes to your pickup configuration output resistance will skew the output balance between your bass and the feedback pickup so you can easily tweet the intensity of the effect with your volume pot or if you have coil splitting going on. When you get the hang of it, it is possible to create some pretty interesting and potentially useful affects. If you're one of those vintage purists out there, it doesn't get any more analog than this. Now please, share your thoughts, or simply tell me I'm insane.