SDB-4.1 Dual Source 4 Pair Speaker Selector with Zone Volume Controls The SDB-4.1 is a highly efficient dual source, impedance-matching speaker selector. This high quality speaker selector is designed for use with either 4 or 8 ohm speakers and includes a volume control for each pair of speakers on the front panel. This volume control allows you to set and forget the pre-amplifier/receiver volume and use the volume controls on the SDB-4.1 to control the listening level in each room from a central location.