In the words of Spaceman Effects:

From transparent edge-of-breakup overdrive to thick sustain-laden wall of fuzz, the Rumblefuzz II offers a wide palette of harmonic distortion. This pedal acts as an extension of your instrument, with a natural dynamic interaction to your playing and instrument controls. True to its name, the Rumblefuzz II especially shines on bass and baritone guitars, with full round lows and a bite that perfectly cuts through the mix.