KEF Q400B Front Firing Powerful Subwoofer - Black Ash (Single) The Q400b features a closed box design housing a 10 inch bass driver, driven by a 200W on-board Class-D amplifier. This subwoofer creates spectacular bass imagery that eloquently fills any room with undistorted, quick-responding bass. Anchoring your whole sound performance in an all-embracing sensation of immediacy, the Q400b completes your acoustic picture. The Q400b features rigid, new aluminium cone technology very much like our Uni-Q driver array, making the subwoofer a tour de force in its own right. This powered subwoofer is constructed to prevent unwanted resonances that could muddy up your sound, all while efficiently delivering bass at all the right times.