Perhaps I can shed a little light here. You will need a stable 4 ohm load bank and not the speakers themselves. Speakers are a reactive device and not a purely resistive load. So you will need a solid resistive load to get an acurate measurement. Next you need to check and make sure your meter is frequency compensated. Most are not. Lastly you will need a CD with test tones on it. Music is all about the dynamics and you will not get an accurete ready.

Connect the load bank to the output of the amp, bring up the volume a test tone, probably something around 1Khz and measure the voltage at the speaker terminals. Then it is just a little math to get the power output.