Two adjust way: adjust by gain, adjust by frequency | Adds subwoofer level controls or balances a multiple amplifier installation. Just need to rotate the button of the controller, easy operate. | Adds subwoofer level controls or balances a multiple amplifier installation. Just need to rotate the button of the controller, easy operate. | Different frequency can make the song feel different. Universal fit for most of car, simple in and out connection, no additional wiring need. | Just need to rotate the button of the controller, easy operate.