The 21S SUB is a premium subwoofer engineered to provide robust low-frequency performance in permanently installed sound reinforcement systems. Designed to integrate seamlessly with the xS Series and xA Series loudspeakers, the 21S SUB is well-suited for a variety of professional environments including theatres, houses of worship, multipurpose venues, and corporate facilities. It houses a single high-excursion 21-inch driver within a bass-reflex enclosure, optimised to deliver substantial sound pressure levels while maintaining a compact cabinet profile. The inclusion of a specially shaped reflex port enhances airflow and minimises distortion, enabling consistent and reliable performance at high output levels. When operated in INFRA mode, the 21S SUB significantly extends the low-frequency response of a system down to 33 hertz, offering greater depth and control in demanding acoustic applications. Designed with flexibility in mind, the 21S SUB supports both ground-stacked and flown configurations, whether installed individually or in pairs. Constructed from marine-grade plywood and finished with a durable, impact-resistant paint, the enclosure is equipped for long-term use in professional installations. A front-facing metal grille with acoustic foam backing ensures component protection without compromising audio quality. The Weather Resistant variant, rated IP55, supports reliable outdoor operation and includes a fixed cable. The 21S SUB is a dependable solution for enhancing low-frequency performance across a range of installed audio systems