The S4S flush-mount passive subwoofer is part of the Synthesis Four system, offering you a choice of ways to fill 7,500 cubic feet of space with dramatic, full-spectrum sound from compact system components. Begin with three-way, in-wall main loudspeakers (one S4HC and two S4VC’s). Back them up with two S4S subwoofers and an S820 amplifier for an all-in-wall solution and four S4Ai in-wall multipole surrounds. Add the power of an SDP-40HD processor/controller, SDEC-3000 digital equalizer and S7165 7-channel amplifier. A complete S4S loudspeaker system includes the S4S in-wall subwoofer and the S4SBB back box, which must be purchased separately. A high-quality, external two-channel power amplifier, such as the JBL Synthesis S820, is also required, and must be purchased separately.