How to choose the right speakers for your amplifier or AV receiver

Impedance: Measured in ohms (Ω), typically 4, 6 or 8. The lower the impedance, the more demand the speakers place on the amplifier, which is why matching the impedance of your speakers to your amplifier is important. Most home theatre speakers will be 6 or 8Ω, as are most home theatre amplifiers. Hi-Fi (2 channel) amplifiers are often capable of handling impedances of anywhere from 8Ω, all the way down to 2Ω. Consequently, speakers designed for 2 channel systems come in a wider range of impedances.
If, for example, you connected 6Ω speakers to an 8Ω amplifier, they would put more of a load on the amp than it’s designed to handle, and produce less than dynamic (clean, effortless) sound. You then run the risk of the amplifier overheating and shutting down, or worse, burning out altogether. This will only happen after you notice distortion in increasing amounts though. Thankfully, most modern amps and AV receivers have protection circuits, thermal fuses and the like, to shut them down in the event of overload.
A common complaint of people with mismatched speaker/amp combinations is that they shut down during loud action scenes in movies, or after 2-3 minutes of music at a moderate level. The amplifier/AV receiver often gets unfairly blamed in this scenario.
Power handling: An often misunderstood specification. Measured in watts, it is best represented by RMS (root means square.) This figure tells us how much power the speaker can comfortably handle for a sustained period of time. Also referred to as continuous, or nominal power.
Some speakers only list a peak power rating. This can be measured in many different ways. It’s essentially a rating for the mechanical limitations of the speaker, and often represents how much power a speaker can take before it produces audible distortion. Depending on the type of signal being played back, the speaker may reach its mechanical limits at well below its rated power - for example, bass-heavy movie soundtracks and extremely high frequencies will challenge a speakers mechanical ability. For this reason, power handling doesn’t necessarily translate as a speakers ability to produce sound as many would have you believe.
PMPO (peak music power output) is essentially the measure of the speaker operating at or near its absolute limits on the verge of serious damage. It is not a useful rating, and easy for manufacturers to inflate - so avoid it completely.
Often speakers will simply say "power handling" and a number measured in watts - this may mean peak or continuous. If you’re ever unsure, ask the manufacturer/sales person the question.
When power matching speakers to an amp, a good rule of thumb is to power them with a little more juice than they’re designed to handle - around 10% should do it. That way the amp doesn’t have to work as hard to drive the speakers to their full capacity, resulting in cleaner, more dynamic sound. Speakers are designed to handle fluctuations in power levels, so this extra bit of juice won’t cause any issues.
Sensitivity: Sensitivity is a rating often listed for speakers but not terribly well understood. Measured in decibels (dB), it refers to the speakers ability to turn the power from the amplifier into sound. The higher the rating, the more efficient the speakers are. Anywhere from 87-93 dB is typical of most speakers, but anything 90 dB or above is a good rating.
With high-definition audio formats reaching the home on Blu-ray and HD DVD, demands on speakers are greater than ever before. The dynamic levels and savage transients that formats like DTS-HD Master Audio and Dolby True HD deliver are best handled and enjoyed through speakers that are matched properly to the amplifier or AV receiver that is driving them
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