power

It was explained to me this way many years ago and I always reflect. My previous understanding was that amplifiers provide power to the speaker to use. It was best to get an amp that could match the power rating of the speaker. Now I think that the speaker DRAWS power from the amp. At different peak frequencies it will draw more or less. Also the more complex the music becomes. The amp one chooses must be up the task of provided all the power the speakers demand. If not, it will strain the amp and that can lead to distortion. The best amps have enough current to provide the power needed for the speakers demand. Be aware some speakers dip to a 2 ohm load or lower. Quality amps never sound strained to me. I always pick amps of high current capability, even if they exceed the power requirements of the speaker. To me, good quality watts means less distortion. This comes from quality parts, smart design and well built (often large) transformers. Anyone out there with electrical engineering backgrounds, please feel free to correct my simple understanding here.

The key is, how can you know what "enough" power is? I read an article recently that discussed the power requirements of transients. It could be a quick slap of a snare drum or a tympani, really any sound that is instantaneous can cause the speakers to try to accurately reproduce the sound. Those transients can cause the speakers to draw a huge amount of power in a very short period of time. The ability of your amp to handle these transient peaks will affect the overall sound quality.

My speakers are 4 ohm rated at 96db which is very efficient. I recently traded amps (solid state) and went from 220w per channel which drove the speakers very well, to an amp of the same brand at 470w per channel. The difference in sound quality was amazing. I couldn't believe how much better defined everything was. The bass was tighter, the mids clearer, it was as if I had purchased new speakers. So back to that article; the author noted that due to the high power requirement of transients, the more power the better even with efficient speakers. 

This is a key problem with speakers - especially audiophile designs. At higher levels they compress and sound dull and flat. It is a problem in Xmax excursion of the drivers as well as overheating issues prevalent in small voice coils (less than 4 inch diameter). It is extremely expensive to make good drivers that still sound great when pushed hard and therefore designers most often choose cheaper parts (cheaper speaker parts can still sound great at lower levels but the sound falls apart when really pushed). Note that speaker efficiency is only measured at 1 watt so it does not tell you if the speaker can play very loud cleanly (lower efficiency designs with better parts may play much louder than high efficiency designs with cheap parts).

A very powerful amplifier is actually safer for the speaker as clipping at lower power can damage tweeters more quickly than clean signals at higher power. As Falconquest notes above - 4ohm speakers are much harder to drive than 8 ohm and will benefit from a beefy amplifier that can handle extreme levels of current necessitated by the very low impedance....

Soundstage speaker measurements show a test at 90 dB and at 95 dB and many audiophile designs show compression and high distortion levels at a mere 95 dB - so transients more often than not are a problem that the amplifier cannot fix as it is an inherent limitation of the accuracy of the speakers themselves. Here I am referring to high end audiophile favourite designs like Wilson etc.

Also something to keep in mind looking at amps, not all watts are "created equal". Make sure the amp you're looking at has a stiff well designed power supply not just a high enough watt rating.