As pointed out in one of the threads, when the maximum power of an amp is reached, the distortion rises very quickly. It is a "bell" curve rise. The distortion goes from "zero to a hundred" really fast (smallest movement of the volume control), and the distortion created plays havoc with the tweeter/s and midrange speakers.
Not so much with a group like this, but what I saw with the "how loud will it go" crowd was, they "cracked it" and after a while the tweeter went. Five to ten percent of the sound was lost, so they "cranked it" again. A short time later there was less sound again because the mids were fried.
Woofers pretty much have to be abused with brute over powering or "bottoming" them out with excessive bass and power. I had knuckle heads complain that their speakers were making a "whip cracking" sound, or a "blatty" sound. As it turns out, they have the loudness button in, bass control cranked to the max, and the volume control at 3:00 (because that's only 3/4's of the way up, it should be able to do that).
In my experience, less than 5% of the time did speaker repairs result in a woofer failing.
To go along with that, I saw more blown speakers caused by them being under powered, than over powered, and that goes back to how the distortion increases when full volume is reached..
As for the guitar amp being over driven, in most cases you are dealing with a speaker that is basically a woofer. The distortions being generated by the "stomp boxes" are in the mid and upper range frequencies. This is not really taxing or putting the speaker in any danger.
