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Subwoofers come in two varieties--powered (active) and non-powered (passive). Active subwoofers have their own amplifiers. Many loudspeakers today have integrated passive subwoofers. The problem with passive subwoofers is that large coils are required. These large coils become so resistive that they make the entire system inefficient. The resistance takes away the amps dampening factor. It also creates bass that is slow and uncontrolled. It is very difficult to produce a passive subwoofer that crosses over lower than 100 hz for this very reason. Some of the world's most expensive loudspeakers use passive crossovers. They do this because it's cheaper--not because it is better.

The advent of home theater has made subwoofers very popular. Most of the subwoofers sold separately are active. Most of them use amplifiers made in Taiwan and China that are rated anywhere from 75 to 150 watts. Unlike the amplifiers you use for your full range loudspeakers, there are no standards for how you rate a subwoofer amplifier. Most of these amps would be a fraction of their rating if they had to comply with FCC requirements. The nOrh subwoofer amplifier is rated at 150 watts from 20 to 20,000 Hz. The amplifier is designed as a high fidelity amplifier with an active crossover. It is like buying a high-end amplifier to drive a single woofer. In fact, the nOrh subwoofer amplifier is essentially the same as a bridged Multiamp.

Adding a subwoofer should not shake the walls. It should not vibrate your furniture. It should simply add depth and intensity to the music. When there are sections with deep bass, the bass should create shockwaves that you feel but don't hear. The sounds you hear of things rattling in your house is not from the bass. It is from the resonances (distortions) that cheap subwoofer amplifiers put out.