Louder sound from your subwoofers doesn't always mean adding more power. Amplifiers are expensive, and it's wise to take steps to make your subs louder before assuming that you need a bigger amplifier. Simple adjustments can help you get more and better bass without spending more money on equipment.
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Put your subwoofer in a corner. Placing the subwoofer in a corner can provide up to six decibels of bass boost without the need for more amplification.
Make sure the subwoofer is in phase with the rest of the system. Most subwoofers will have a phase switch that is either continually variable or will offer a 0 and 180 degree position. Play some bass heavy music and adjust the phase switch until you hear the loudest bass.
Move the subwoofer to another location in the room. As a general rule, try to avoid placing the subwoofer an equal distance from two walls. Placing the subwoofer in an asymmetrical position relative to room boundaries will result in louder bass output and the smoothest overall response.
Fine tune the subwoofer crossover controls. An improperly adjusted crossover can cause your main speakers and subwoofers to compete in the bass frequencies and cancel each other out. By properly adjusting the crossover for your particular system, you will get louder bass than by just randomly setting it or leaving it at the default factory settings.