This is a vintage Electro-Harmonix Mole Bass Booster. The Mole works exactly as advertised--it boosts the bass. Can add massive boom to a bass guitar or drive a tube guitar amp into wild amounts of thick, powerful overdrive.

 While the unit is not designed to cut the treble, most players report experiencing a treble cut when the Mole is cranked--the boosted low end more than drowns out the unaltered clean signal that is still passed through the device.