For those who don't really grasp this (and you know who you are! :-)), a n amplifier/speaker impedance mismatch doesn't mean that any damage automatically happens, just that the possibility increases.

If a speaker's impedance is higher than the 'nominal', it merely means that, at a given volume-control setting (which determines output voltage), the current will be lower, and thus the power.

This means that one will be inclined to increase the volume, which means that the amplifier may 'clip' (reach the output transistors' maximum conductance point) while trying to deliver more power.

However, the amplifier will run cool, since the transistors will be carrying less than their design current, and the power supply's rail voltage shouldn't sag with load peaks since the current demand is low.

Now, if the speaker's impedance is a great deal lower than the amplifier is designed to drive, then, at higher power levels, the transistors and power supply can be stressed beyond their capacities.

An amp rated at 100 w/ch will supply this much power into any load impedance. What matters is the output voltage at this power level, which varies with impedance, as simple Ohm's Law describes.

As long as the power level is kept within limits, the amplifier will drive any speaker impedance just fine. Monitor the heat output; that's where overloading an amplifier shows up first.