First a mini crash course in speaker sound. Speakers have been able to produce the high frequencies without a problem, but lower frequencies are much more difficult to produce. Even with a good powerful receiver, the lowest levels of sound force the receiver to put out more power than it has available, and thus neglect the higher tones which ironically, it would be much better at achieving. The larger the driver and amplifier, the lower and louder the sub. It is important to know that the power and size of the sub are not the only thing that makes it good.
Control is also very important.
With such a large driver being moved by such a large amp, it is possible for the unit to misfire and cause a beat to skip or come early. It will vibrate or boom too much. This is the distortion of the subwoofer. Many subwoofers have tricks to gain control over this problem. Velodyne has a device called an Active Servo Control that uses an extra magnetic field to control the subs behavior.