The passive subwoofer XB3 is intended to complement all monitors in the BB5 series in a surround monitoring system. Much of the driver and enclosure technology is identical to that of the XB2, with upgrades to a 15" Radial™ driver and a larger, more expansive enclosure.
As a sub-bass unit, the XB3 has no rival, it's simply the best studio .1-channel monitor to its credit.