DMM - set to A/C voltage setting - the positive probe in (+)speaker terminal and negative probe in (-)speaker terminal
Amp Clamp - set to A/C amperage setting (peak hold if you have it) Put clamp on (+) speaker wire.
Play a frequency to see what max RMS Watts (actual max NOT boss ratings type deal here) you're amp is pushing on your system - 40hz is typical
If you want to do on music play whatever you want.
VOLTAGE ÷ AMPERAGE = OHM LOAD (this is where you see your box rise)
VOLTAGE x AMPERAGE = RMS Wattage
Need a volt m