It depends entirely upon the rating of the individual speakers and how it is all wired.
If wired properly, it's actually fine to run a 100w amp into an 80w cabinet, but you have to be VERY CAREFUL with the volume. Basically, it means that you can't turn the amp up as high as you could if you used a higher-rated cabinet. Simply because the amp has the ability to overdrive the speakers does not mean that it will automatically do so.
For instance, I have a 25w Celestion Greenback in
my Peavey Classic 30. The amp has the ability to overpower and damage that speaker, but I don't let it! I just make sure that I never drive the power amp beyond roughly 75% of its maximum output.
Somewhere along the line, I think people picked up the idea that a 100w amp is always putting out 100w, or a 30 is always putting out 30. The actual wattage being sent to a speaker actually varies quite a bit. With a 100w amp, during normal chords it is probably putting out somewhat less than 100w, but when palm muting, it is over 100w by a good bit.
Most of the time the wattage listed on something, whether an amp or a speaker, is the AVERAGE wattage, not peak wattage. A 100w amp at maximum power averages 100 watts. Sometimes it puts out more, sometimes less. A speaker that is rated for 25w can handle a signal that averages at 25w, but it can also handle occasion peaks above 25w, and still functions when below 25w.
The average wattage (also generally called R.M.S. Wattage) is just used as a rough guideline for matching equipment power levels. It's the average level at which the product was designed to operate optimally at. If you match a 100w head with an 80w speaker, you just can't turn the amp up as high, or it will start damaging the speaker.
Make sense?
