wow, this is all making sense now. So, the speaker has a maximum distance that it can travel - maximum excursion, or XMAX - and if the speaker is driven BEYOND that distance, it rattles against the frame/basket, or whatever it is called that holds the coils, which, if persisted, will likely result in destroying the speaker, by either destroying the cone or melting off the insulation and shorting out the speakers. This is NOT to be confused with the clipping of the amp - which to the extent of my knowledge is what causes distortion, at least when combined with some compression.
I think I've had that happen with a couple cabs I've had -
1) with an old regal amp, and
2) with a 110 practice cab that I have made with a fixed up radioshack speaker
If I put the regal amp past 5, which was the point when it actually started sounding off very much at all, the speaker would make this terrible rattling noise. I then decided to make the 110 cab, which has a slot that fit my warwick 125 watt head. I tried using it a little for band practices, but it wouldn't compete well enough and when pushed loud enough it would also rattle, which is weird because the speaker was rated for 125 watts.