|Elliott Sound Products|
This is the old "how does clipping blow up a woofer" problem. I know that woofers rarely blow up, even with severe overload, and in essence they seem to be immune (although that might overstate things a bit!). This is (or would be) true, but for one small detail. Few speakers (including woofers) can tolerate their full rated power on a continuous basis. The rating is more for "amplifier power" than speaker power.
Imagine that a 100W amp is just on the verge of clipping with a full range or band limited signal. Although the amp will be peaking at 100W, the average power will be closer to 5-10W, or maybe a little more or less depending on program material. If you push the amp harder, the peaks will clip, which you might not even notice if the mid+high is still clean. The average power may be 50W or so now, and the speaker's voice coil will start to get quite hot. Push it harder again, until even the lower level signals are clipping too, and the average power is now well over 100W.
Note that a 100W amp driven with a square wave or severely overdriven sinewave will give close to 200W. This problem is worse with semiconductor amps than valves, because valves have inherent inefficiencies that reduce the maximum output.
If this is kept up for too long, the voice coil will literally burn up from the heat, the adhesive lets go of the voicecoil windings, and the whole assembly starts to disintegrate. Exit one woofer.
Having said all this, it still doesn't happen too often, but generally you are better off with an amp that is rated for more power than the speaker can handle, rather than less (except for guitar, but guitar amps are driven into clipping a lot (most) of the time).
There are - naturally - limits. A 10W woofer won't last long with a 300W amp driving it at anywhere near full power on transients, but a 100W woofer will last forever with a 10W amp in permanent overload. Some common sense must be applied, but a typical 100W woofer will probably last forever with a 200W amp used sensibly.
The comment above is aimed more at the risk of tweeter damage, but it can happen with midrange drivers and woofers too, given the right (or wrong) set of circumstances. One issue with many modern recordings is excessive compression, and this can increase the average power level dramatically, even before amp clipping.
It is worth noting that tweeters are rarely blown up just by the additional harmonics generated when an amp clips. Although they are undeniably present (and sound undeniably terrible), the actual power of the harmonics is not as high as you may have been led to believe. It is the combination of these harmonics and the "compression" effects described above that do the damage, and clipping compression is the greatest offender.