Measuring impedance with multimeter

You can measure speaker impedance using Room EQ Wizard and a home made harness.


https://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html


I personally recommend DATS V2 as a complete, out of the box solution:

http://www.daytonaudio.com/index.php/dats-dayton-audio-test-system.html
Around $99 at Parts Express

As said above, the 'resistance' that you measure with your ohm meter is in DC, i.e. direct current. Impedance is an entirely different measurement that uses an alternating current (The music signal happens to be just that, AC). It might like understanding the horsepower of a car. The stated horsepower isn't exactly what you will be experiencing in driving, at least considering differing speeds, roads, driving habits, etc. It gives a baseline for you to consider. 

If its the m3 and its $300 in Eminence drivers that DC of 4.2 is about right. 4.2DC is about 8 ohm average. If you want a 16 ohm loudspeaker front horns and a few Fullranges are about the only choices I can think of off hand.