Electro Harmonix Bassballs Twin Dynamic Envelope Filter (Second-Hand)

The Bassballs from EHX is a an envelope filter specifically tuned for bass guitar. Utilising twin-tuned filters, the Bassballs really captures a wide variety of envelope sounds, ranging from the super subtle, all the way to dressing up like Bootsy Collins. 

This example is the version just before they moved the circuit to the nano format. These units are also quite useable with guitar as well. A notable user is Mick Thompson from Slipknot,  using one one on the track 'Disasterpiece'. 

Fully functional, and includes the original box.