Hi Bob,
Here is a quick explaination. I work for a speaker company, pro audio, PA's etc. We build our own drivers, and in some cases we do make essentially the same driver with different impeadance values, 2,4,6,8 & 16 ohm. There are tradeoffs to be made. Higher impedance drivers have lower sensitivity, i.e. the amount of power you put in results in lower power out as your sensitivity goes down. On the plus side of the equation, lower impeadance drivers can handle more power. Axiom has choosen to design their product to provide good sensitivity and high power handling, with good frequency response. It seems to be agood mix, my M80's sound great.
This is due to the voice coil wire. Thicker voice coil wire allows more electricity to flow through it more easily than a thinner wire(lower impedance), and it can take more heat for a longer period of time, resulting in higher power handling. But it has more mass so it can reduce sensitivity. There are a lot of other factors involved (cone mass, surround & spider stiffness etc.). The general rules above aren't true in every case, but I think it illustrates the idea.
