MadScientist is essentially right. But good luck finding that "stable" load bank. By stable, he means that it can't change resistance as it heats up (I suppose you could rig it up in your freezer
). This is what I mentioned earlier about the testing that Emo is doing.
And he's also right about the speaker not always being 4 Ohms. The impedance changes with frequency. Now how much it changes I don't know. And he's also right about a frequency-compensated meter. So, you could go about this in two ways to get a very gross indication:
1. Put on some pink noise. Avoid a test tone as you could end up over-heating and damaging one of your drivers!!! Ask Axiom to tell you what the impedance of the M80 is with pink noise. I wouldn't be surprised if it's somewhere around 4 Ohms. Set your volt-meter to 60V AC (not DC). Find some way to clamp the leads on to the speaker terminals (DON'T ATTEMPT TO HOLD THE LEADS AS 15 THOUSANDTHS OF AN AMP IS ALL IT TAKES TO KILL YOU!!!). Turn up the EMO and take readings of the voltage. Your power will be E squared over the speaker impedance.
2. If Axiom won't give you the impedance, then you have to measure both the voltage and the current. So you will need two meters set on AC. The voltmeter will go across the terminals as in 1. and the ammeter will go in series with the wires. Set the ammeter to 15 amps. Clamp the ammeter on (DON'T ATTEMPT TO HOLD THE LEADS AS 15 THOUSANDTHS OF AN AMP IS ALL IT TAKES TO KILL YOU!!!). Take both measurements and multiply.
Take SPL readings as well if you can. I have to tell you that I really don't know what kind of accuracy you will get from these measurements as it all depends on the meter characteristics. Remember: DON'T ATTEMPT TO HOLD THE LEADS.