The Fosi Audio M04 is a compact 100-watt Class D mono amplifier engineered for versatile bass and full-frequency applications. Its dual-mode SUB/PBTL switch enables seamless operation as either a dedicated subwoofer amplifier or full-spectrum mono driver, accommodating both active and passive subwoofers alongside passive speakers. The integrated power control minimizes footprint while maintaining robust output capability, delivering the low-frequency extension necessary for immersive cinematic and musical experiences. Whether configured for in-wall installation or standalone placement, this efficient amplifier provides substantial bass impact in a remarkably compact chassis—proof that size need not compromise performance.