Only you can answer this question, but we can certainly explain why you may or may not need one. Basically, if you feel you are not getting enough bass from your system, you can use a sub.
There are several advantages to adding a sub to your system.

If you have your bass tone control turned up past the 'flat' (0dB) mark, adding a sub will enable you to turn the bass control back down to 'flat'. This will result in your receiver amp not having to work so hard. It will also stop your speakers from having to reproduce all of that extra bass, extending the life of both the speakers and receiver. It will also simply improve the sonic quality of your system. If you currently find yourself pushing your system very hard (loud) on a fairly regular basis, adding a sub will help a lot. A powered sub adds a lot of power to your system, bringing the overall volume of your system up. You will find that you don't have to turn your system up quite as loud to get the desired results. Of course, once again, you will get better sonic quality and this will extend the life of both your speakers and receiver.

If your receiver allows you to set your speakers to Large or Small, in many cases you will want to set your speakers to Small even if they are Large. This will prevent the speakers from reproducing the subsonic frequencies, once again giving you better sonic quality and extending the life of both your speakers and receiver.

Now, there may be some drawbacks to adding a sub, but to be honest, we can't think of any at the moment.

THE OPTIMUM SET-UP 1. Rectangular room (example: 10' x 12') 2. TV is placed dead-center on the front 10' wall with the center speaker sitting on top of it and pointed at the listeners head. 3. The couch is placed 2' from the back wall, facing the front wall, centered between the left and right 12' walls. 4. Being that the couch is now 8' from the front wall, the front left and right speakers are placed 8' apart on the front wall with the TV centered in-between them. This is a general rule. If the couch were 10' from the front wall, you would place the speakers 10' apart, and so on. The tweeters of the two front speakers should be at ear height. If they must be higher, they should be aimed at the listeners head. 5. The rear speakers will be placed on the left and right 12' walls, directly in line with the couch (2' from the back wall) and aimed at each other. They should also be placed very high. Approximately 6 - 8 feet from the floor. 6. The subwoofer will go somewhere along the front 10' wall. Wherever it sounds best. Placing the sub in a corner will usually allow it to project better. Of course, this is the Optimum design. Get as close to this as possible, and you will have the best sound you can get in your room

When a speaker begins to pop, it means that it is receiving a signal that is "clipped" or "distorted". This is telling you to turn down your volume control. If the bass or treble controls are set too high, they will cause distortion when the volume control is turned up past a certain point. This, once again, is usually the result of "under-powering". Your tone controls (bass and treble) are actually volume controls except they affect only a specific section of the overall frequency range. For example: If you turn up the bass control on your receiver, you are increasing the volume of the low frequencies only. All else remains the same. Now you also turn up the volume control. You have now increased the volume of all frequencies, including the bass frequencies that have already been increased! The bass is now so loud that it is beginning to distort when a loud burst of sound is reproduced. When the speaker pops, this is what you are hearing. The same holds true for the treble control and your speakers tweeter. Permanent damage can result under these conditio

There are two ways to blow speakers. The first is the one we all know and understand. If you pump 500 watts into a speaker that is only rated at a 100 watts, kaboom, it blows up. As a result, very few of us do this. So then why are all of these poor, helpless speakers blowing up? Because of the second reason, "under-powering". It's really a misleading term, but let's try to explain. First let's take it to the extreme. Example: You have a 1000 watt speaker and a 100 watt amplifier. The way you like to listen to music requires 300 watts. Typically, you will ask your amp to put out the 300 watts that you want to hear. The amp will do it, but it will "clip" and "distort". Unfortunately, the human ear can't hear distortion until it reaches almost 10%. By that time it's usually too late. The speaker keeps trying to reproduce this distortion that it's not capable or reproducing and eventually, it blows (usually the tweeter fails first). There is a very easy way to determine if you are asking too much of your amplifier. Follow these directions to perform this test. Set the Bass and Treble knobs in the middle (flat).Make sure all Equalization buttons are turned off (loudness, EQ, contour, etc.)Put on a CD. Turn the volume half way up (electronic volume controls, display would read 0 dB). If you normally listen louder than this, chances are you are pushing your amp too far, causing it to clip, and blowing speakers. Generally speaking, when listening to CD, VCR, DVD, or Laser-disc, you are at maximum clean listening volume when you reach the half-way point on your volume control. You may be trying to watch a home-recorded VHS or Cassette, and it may not have been recorded very loud. So you need the extra volume control to make up for the difference in a situation like this. Please keep in mind, if you normally listen with your bass and treble knobs boosted and/or the loudness button etc. pushed in, it is possible to push the amp too far with your volume knob as low as 1/3 volume. If you are pushing your receiver too hard, it is suggested that you realize your systems limitations or recommend that you get a more powerful amplifier.

Dipole speakers are used as rear channel speakers in THX Certified speaker systems. They contain two sets of speakers, both of which fire both front and rear, and are wired in phase with each other. This creates a diffused non directional sound field with a null at the listener's position. Bipole speakers are similar to Dipole speakers. Instead of sharing the same chamber, front and rear chambers are created with a dividing board isolating front and back speaker sets. They are wired out of phase to create a delay effect between the middle and rear portions of the room. Bipole speakers can be used with Dolby Pro-Logic and Dolby Digital systems. They are highly recommended for use in DTS systems.

This subject is a matter of personal taste. It is entirely possible that you may like a two-way speaker better than a three-way. Your best bet is to listen and decide which sounds better to you. Not by how many components are in the speakers, but by how they sound. In a Three Way speaker, the woofer, midrange and tweeter each have a specific bandwidth of music to reproduce. The low frequencies to the woofer, the mid frequencies to the midrange and the high frequencies to the tweeter. A Two Way speaker needs to divide the mid frequencies between the woofer and tweeter. Because a Three Way speakers midrange driver, the mid frequencies are sometimes more pronounced than with a Two Way speaker. There are also Four Way (Woofer, Mid-bass, Midrange and Tweeter) and Five Way (Woofer, Mid-bass, Midrange, High-Midrange and Tweeter) Speakers.

Here are a few tests to perform if you find that one of your speakers has stopped working. Be sure that the problem occurs with all signal sources (CD, VCR, Tape, etc.). If it only happens with one signal source, chances are that the problem is with the source or its connection cable. Check to see if the Balance control knob is not turned fully left or right. It should be in the center located by a noticeable notch felt in the control knob. Most surround sound receivers have a Test Tone Generator in order to test and balance all of the speakers. If all the speakers test, then the problem will most likely be in the receiver. If the same speaker still does not work, swap the malfunctioning speaker with one that is working. If the good speaker works in the position of the bad speaker, then you know that the problem lies with the speaker. If the good speaker does not work in the position of the bad speaker, and the bad speaker works in the position of the good, then on the back panel of the receiver/amp only, swap the speaker wires of the bad speaker position with one that works. If the problem remains with the same speaker, then the speaker wire is bad. If the problem moves to the good speaker position, then there is no output at that speaker terminal.

This usually means that one of the drivers (woofer, midrange, etc.) in the speaker has stopped working. At the back panel of your receiver, swap the two speakers (connect the left speaker to the right channel, and the right to left. If the same speaker sounds thin or muffled, bring your speaker to an authorized service center and have them service it for you.

An Ohm is a measure of impedance or resistance, and is named after the 19th century German physicist Georg Simon Ohm. The Impedance value of a speaker is an average measurement of how much restriction your amplifier or receiver will see from the speaker. A good example would be water flowing through your garden hose. When the spigot valve is fully open, there is no restriction. This would be like taking the positive and negative wires that would connect to a speaker and twisting them together (not something you want to do). Current would flow unrestricted between the positive and negative connections of the amp. Such free-flowing current would cause immediate damage to the amp. Some amps have protective circuitry that will shut the unit off under these conditions, thus protecting it against extensive damage. Some kind of restriction must be placed between the positive and negative terminals of the amplifier's speaker output terminals. This is where the speaker comes in! Back to the Garden Hose example! If you were to turn the spigot valve so that it is one quarter closed, we can equate this to 4 ohms. Now turn the valve so that it is half closed. This is 6 ohms. Finally, turn the valve until it is three quarters closed. This is 8 ohms. As you can see, the greater the restriction, the higher the number. With a higher impedance speaker, the amp does not have to work as hard as with a lower impedance speaker. Its operating temperature is cooler, but it does not put out as much power. Different impedance speakers are designed for different applications. Many stereo amps and receivers give a power rating with both 4 and 8 ohm speakers. Because there are only two amp sections,(left and right) more attention can be applied to the problems associated with 4 ohm compatibility. Heat dissipation is usually the primary concern. If the cooling concerns of a particular amp are taken care of, then a lower impedance speaker can be used to get the most out of it. However, in the case of most Dolby Pro-Logic and Dolby Digital/DTS components, there are five amps inside the unit! This makes for a lot of heat. Because of this, most Dolby Pro-Logic and Dolby Digital/DTS receivers are only stable to 8 ohms. Always follow the receiver manufacturers specifications and recommendations for appropriate speakers.

The Sensitivity or Efficiency of a speaker tells how loud the speaker will play under standardized conditions. You will see a specification that may read "90dB SPL 1Watt @ 1meter". This means that when 1 Watt of power, usually a 1kHz tone, is fed to the speaker, the resulting output is 90 decibels measured at 1 meter from the speaker. The lower the dB rating, the more power needed to reproduce at the same volume as a speaker with a higher Sensitivity rating.