Regarding the resistance measurements. You can't just measure it with a multimeter and get an accurate picture. Impedance changes with frequency response. Take a look at an impedance curve. like this one.

http://www.churchsoundcheck.com/imp1.html

You'll see that the impedance is highest at the resonance frequency. This is because of electro-magnetic properties of the transducer. As the voice coil moves within the magnet assembly, it produces a voltage in the voice coil, this can be thought of as some "backpressure" on the system, that's why the impedance is higher.

Selecting the "nominal impedance" that a company puts on it's spec sheet is a bit of an artform. Some will select the lowest value, others kind of an average, after throwing out the extremes.

re-wiring your M80's is a bad idea.