Overpowering Speakers

You have to double your power to get a 3db gain. It takes 3db to get a noticable difference in volume. If you are listening to a speaker of 90db @ 1 watt, 2 watts give you 93db, 4 watts give you 96db, 8 watts give you 99db, 16 watts give you 102db, 32 watts give you 105db, 64 watts give you 108db, 128 watts give you 111db, 256 watts give you 114db.... 121db is our threshold of pain. This tells you how speaker sensitivity dramitically effects how much power is needed.

Just a quick add on to make the point of sensitivity and power. All other aspects being equal, a 93db sensitivity speaker will play a volume of 111db using 64 watts. A speaker with 84 db sensitivity will play a volume of 111db on 256 watts.
Your speaker choice makes all the difference in the world on how much power is required to hit the volume levels that you seek.

Timlub, I'm pretty sure you will need over 400 watts to drive an 84 db speaker to 111 db.

Here's a useful web page for things like this:

http://www.sengpielaudio.com/calculator-gainloss.htm

take a look at the chart at the bottom of the page.

I appologize, I had just run the numbers off the top of my head, but the formula is fact, not opinion, so
84 @1 watt 105 @256
87 @2 watts 111 @512 watts @ 84db sensitivity
90 @4 111 @ 64 watts @ 93db sensitivity
93 @8 I missed one line in figuring.
96 @16 110 @ 16 watts @ 98db sensitivity speakers
99 @32
102@64
105@128
Thanks Atmasphere, your chart is correct, but I was quoting how much power it took to achieve a 3db gain, not how much voltage it takes to double your volume. My numbers are correct with all things being equal.

In any event-The question was about having an amp that exceeded specified wattage limits the speaker manufacturer stated. And if exceeding that predefined wattage limit would then cause harm?
The answer is yes, - an amplifier rated as having wattage exceeding your speaker's capacity at full output if played "continously" would damage your speakers.
BUT amplifiers typically deliver much much less wattage than full output during normal sane playback. As was already said the real problem is the sudden demand on an amp that doesn't have the "head room" to deliver the required wattage "cleanly" during short peaks or bursts in the music. If your amp is rated as a low output amp, it will be much more likely to deliver electricity (DC) to the speaker which is "clipping" as mentioned. It is this phenomena that suppsed burns up your speaker. I have also heard the limtation in the wave forms where they get cut off and demonstrate non linear highly distorted forms can also to a number on your speakers.