How do the monitor10's have 8ohm rating

Ben:
I'm no electrical engineer, but I can tell you that the 10B's I owned in the 1980's were very efficient, easy to drive boxes. The great sound was the best part, of course.

I can only vouch for the original Monitor 10, which was identified as a nominal 6 ohm speaker in Polk's specifications.

Impedance is the AC analog of the DC quantity called resistance. Like resisantance, it follows Ohm's law: E = IR (voltage = current times resistance or impedance). Unlike resistance, impedance is a complex (in the mathematical sense) quantity comprised of resistance and capacitive and inductive reactance. For the purposes of this discussion, the most important concept is that impedance is frequency dependent. The "rated impedance" of a speaker system is a nominal quantity, usually measured at a frequency somewhat above that of the peak impedance of a system (which occurs at its (low) resonant frequency). There's usually a flat 'valley' of impedance somewhat above the resonance impedance peak. At higher frequencies, above that valley, impedance tends to increase.

Measuring speaker impedance is straightforward but not trivial :-) Yes, you CAN do it yourself, especially for a single speaker driver in free space.
Here's a good tutorial. http://sound.westhost.com/tsp.htm


EDIT: Here is a typical impedance curve of a single driver with no crossover or network for impedance compensation:


The 'nominal' impedance of this driver would be that observed out in the region of about 200 Hz.