Coming back to Doc's original question, if you solve the same equation for I, you get:
I = sqrt[P] / sqrt[R]
This means that the lower the load ohms, the more current the amp has to pump out for a given power output. More current means more heat&stress born by the power transistors in the amp. That is why the amp may eventually engage its thermal or overcurrent shutdown circuit when driving low ohm speakers very hard. Some speakers actually have impedance values dip drastically below its rated ohms, at certain frequencies. Those speakers, especially if they are less efficient and thus gobbling up lots of power, can cause shutdown problems on less tolerant amps.
Luckily, Axioms' impedance curves behave pretty well in general (no huge dips in the power-hungry bass frequencies), according to the NRC Canada measurements available online. Plus, the Axioms are very efficient speakers, leading to the claim that they are "very easy to drive'!
Impedance curve for the M22 (rated 8 ohm):
Impedance curve for the M80 (rated 4 ohm):
Source:
www.soundstagemagazine.com
