Gearooz explains the how to choose and use a subwoofer, in the context of stereo applications.

A subwoofer is required specifically to reproduce low frequencies; it usually doubles up to handle the bass content of all main channels.

Adding a sub to your setup can relieve your main speakers from the task of low-end reproduction which, with cheaper amps or low wattage speakers, can actually help them produce cleaner sound.

Subwoofers can be effective and suitable when used properly. Equally, though, it is very easy to destroy any chance of good audio quality with an inappropriate choice of sub or a bad set-up. Poor subwoofer installations usually suffer from too much, or poorly defined, bass. Often there is a hole in the frequency spectrum in the crossover region between the satellite speakers and the subwoofer. It is therefore becoming increasingly common to have smaller left and right speakers, plus one or more subwoofers — a format often referred to as 2.1. The two refers to the main pair of stereo (left and right) speakers, while the '1' refers to the limited-bandwidth subwoofer.

The worst kind of subwoofer system will only provide a monotone noise of energy, regardless of the dynamics of the bass instrument, and the bass might then appear to be slow or late relative to the main speakers. On the other hand, a well-designed and well-configured system will enable a more transparent mid-range and higher overall output thanks to its low distortion.

Another useful benefit of a subwoofer is the additional power handling presented to the system as a whole. So employing a sub to handle much of the bass power relives that pressure from the speakers, with useful benefits in overall power handling and clarity.

There can be advantages to having two or more subwoofers but generally one is sufficient.

When buying a subwoofer it is important to test it before you take it out on the road. The rooms own acoustic properties are important. If you put the best subwoofer in an acoustically poor room, you'll get poor bass performance! Some subwoofer combinations will integrate better than others so testing it at home will reveal where your pairing is a success or a failure. The subwoofer also needs to have a linear driver; an accurate and powerful amplifier and a well designed and built cabinet.

Bass management is the process of removing the bass element of the signal fed to each satellite speaker, and routing it instead to one or more subwoofers. For example in a simple 2.1 stereo system, this bass management is usually built into the subwoofer, and may be active or passive. There are various approaches to wiring, but most route line-level signals from the controller or preamp to the subwoofer first, which filters the signals and outputs them for the satellites. Some systems work the opposite way round, connecting the signal to the satellite first, and then down to the subwoofer, of course, different systems implement bass management in different ways.

When it comes to placing a subwoofer, there are several things to consider. Although a high-quality subwoofer should not output the higher frequencies that allow its position to be determined by listening, it still cannot be placed anywhere. Sub woofers should be well away from corners, and behind the listener but as equally close to the speakers, you should also avoid placing it at the centre of the room. The closer the subwoofer is placed to the wall, the greater the bass boost will become. Some models are designed to be placed close to a wall, specifically to benefit from this.

Matching speakers with subs designed for them will usually produce the best results. Mixing and matching speaker series may take more time and tweaking, you could end up with inconsistent tones and mismatched cross over points.

Adding a sub will enable you to hear more of the bass frequencies distinctly rather than as indistinct tones. This can help in tuning instruments however some genres won’t benefit as much from a subwoofer addition.  So adding a sub is not just about adding more bass; rather, a properly integrated subwoofer can improve the overall sound of the system.