This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
One reason to get a subwoofer is to add bass to a feeble system.
A second reason is to move the lowest frequencies to a separate
driver, and thereby reduce a particular kind of distortion
caused by the nonlinear mixing of different sounds, called
"intermodulation distortion". A third is to increase the power
handling ability of the system and the overall reliability. All
are valid reasons, but it isn't so simple.
To improve the sound of a good speaker system, a subwoofer must
"integrate smoothly" into the system, extending the bass without
causing peaks or dips. Many subwoofers have a crossover that
goes between your amp and your main speaker which sends the lows
to the subwoofer and sends the higher frequency signals to the
main speakers. This may damage the perfect sound of a good
system, it may sound similar, or it may sound better.
Most good small speaker systems have a bass peak at resonance,
which attempts to compensate for the absence of lower bass.
Like it or not, this is the only way to make a small system
sound realistic. If the small system is done well, the
improvement you will get from a subwoofer will be small, but
still real and, to many, significant.
Correctly done, a good subwoofer will enhance the sound of a
good small-box system. Done wrong or haphazardly, anything is
possible. Even a fine large speaker system might benefit from
careful addition of a subwoofer. However, the better the
original system, the more likely it will be that a modest
subwoofer will do more harm than good.
Low frequencies travel less directionally than high frequencies,
so many people say that only one subwoofer is required for good
sound. This is true to some extent, but not completely true.
There are a few reasons for getting two subwoofers. Some feel
that you need two subwoofers to accurately reproduce the stereo
image, no matter how little low-frequency stereo information
there is. Others feel that two subwoofers are much easier to
set up in a room, less likely to excite standing waves in the
room, and give smoother sound.
A third reason is that two subwoofers can produce twice the
sound of one. Finally, even though subwoofers produce very low
frequency sound and very low frequency sound is non-directional,
subwoofers also have output at 100 Hz, and sound at 100 Hz is
directional, so two subwoofers will give a slightly better
stereo image than one. Assuming, of course, that the two are
separated by at least two feet.
Finally, even though original source signals rarely contain any
music with stereo components below 50Hz, there may be some noise
component with low-frequency out-of-phase noise. This unusual
noise might add a sense of space to a recording if it is
reproduced by a system in which the woofers are very far apart.
It is still true that a single good subwoofer, correctly added
to a system will help the sound but two will probably help more.