The S15 and S18 sub-bass systems can be used in conjunction with the F12 and F15 models to broaden and enhance low frequency output. The S15 sub-bass produces a low and dry sound thanks to a 380mm driver, while the S18 sub-bass, thanks to the 460mm driver, offers a very low and deep sound. Both drivers feature a 101mm high-temperature voice coil, the cabinets feature large reflex chords to reduce airborne noise. Each sub-bass system can operate passively, via a switchable internal passive crossover, or active, using the M3 controller. Both the S15 and S18 systems have a top attachment for installing an F12 or F15 diffuser.