Electro-Harmonix Octave Multiplexer
For your consideration, Garrett Parks Guitars presents this Electro-Harmonix Octave Multiplexer.
From the EHX website:
Get down! Generates deep, phat bass tones one octave below the notes you play. Two separate smoothing filters enable you to tailor your sub-octave signal to the exact bass sound you desire, separating the Octave Multiplexer from other octave devices. Run your vocals through it and sound just like Ike Turner. This is a monophonic device and works only on single notes.