Power ratings are one of the hardest things to measure and communicate since a speaker's ability to handle power is totally determined by the kind of signal going into the amplifier and how close that signal is to "typical music". If you take a 100w amplifier and run a single, high frequency tone through it with the amp set to deliver 100w into the speakers you will probably vaporize the tweeters.
If you run a typical music signal (with a distribution of high and low notes, and varying loudness from moment to moment) and set the amp so that it is delivering 100w at the loudest peaks it will sound fine (although you may get told to turn it down
).
When you adjust the volume control on a receiver or amplifier you change the voltage of the audio signal being sent to the speakers. When you send more voltage to the speakers more current flows through the voice coils and you get more power being delivered to the speaker. The speaker transforms electrical energy into acoustic energy so more power in = more sound out.
You can use a high powered amplifier with any speaker -- it's just that you have to be careful with smaller speakers so you don't give them too much power and overheat the voice coils. With bigger speakers and higher power handling capability (like M80s) you don't have to worry so much.
Most people here (including me) will tell you that you are more likely to damage speakers by overdriving a low power amplifier than by using an amplifier that is too powerful.
I think most people run M80s with receivers in the 100W range, so strictly speaking their ability to play loud is limited by the amplifier power more than the speakers. They generally don't care, because the REAL limit on playing loud is generally either family comments or your own tolerance for loud music, and that limit usually gets hit before the amplifier runs out of power
If you have a large room and the ability to play and enjoy loud movies/music (I'm talking SERIOUSLY loud here) then a more powerful amplifier (200-400W) will let you play more loudly without distortion. Many people also feel that the more powerful amp gives them better sound when playing at lesser volumes, although this is hotly debated by people who don't own more powerful amplifiers
I would interpret the 400W power rating on M80 to say "if you have a power amplifier of about 400W/channel playing typical music, the amplifier and the speaker will start to hit their limits at approximately the same time". You don't *need* a 400W amplifier; a better statement would be "the M80s can make good use of a 400W amplifier but most people don't need that much power".
As others have posted, Axiom does test M80s with more powerful amplifiers on a regular basis, but the testing is done in an sound insulated room because it's just TOO DARNED LOUD otherwise.
On the other hand, the 4 ohm rating *is* something you need to consider. Remember I said that the volume control affects the voltage being delivered by the amplifier ? For any given voltage (volume control setting) a 4 ohm speaker will draw twice as much current from the amplifier as an 8 ohm speaker, so the same volume control setting will deliver more power into M80s than into (say) M60s.
The biggest discrete amplifiers are designed to cheerfully handle a 4 ohm load, as are the amplifiers in receivers from HK, Denon and a few others (Rotel ?). If you have an amplifier that is not happy with a 4 ohm load, the "overload protection" circuitry will kick in too early and either shut down the amplifier or limit the power you can get. In other words, those amplifiers would be able to deliver more power into an 8 ohm load than a 4 ohm load while the better amplifiers can deliver more power into a 4 ohm load.
Confused yet ?