A 2 x 21 high-level subwoofer system" typically refers to a professional audio speaker system designed for delivering powerful and deep bass frequencies in sound reinforcement applications.

It is Proven choice for Large scale Events. 
• Model: HLS2121
• LF- Driver: 2 x 21 inch
• Power Passive: 2000W + 2000W
• Voice Coil: 5 inch

Passive Subwoofer 
2 x 21 inch 4000W High Level Subwoofer System
Digisounds HLS2121