What both recent posters said is correct: If you have subs that can handle max output of your amp, then you set your amp's output voltage with a DMM after inputting the amp's max rated power into the formula outlined on the sticky (and several places in this thread);
if your subs have a LOWER rated WRMS than the max output of your amp, then you calculate using the sub's max WRMS rating, which is referred to as gaining-down your amp.
Ex 1.) Sub has rating of 1000WRMS while amp will only output max of 760WRMS into 4 ohms. Via the formula, sqrt(760*4) = 55.1, so you'd set the amp to output 55.1V.
Ex 2.) Same amp, same sub impedance, but this sub can only handle 500WRMS. sqrt(500*4)=44.7V. (So you are maxing your driver, but gaining-down your amp to do so)
Colossus: I didn't check to see if the OPTI is rated for running at 1/2 ohm, but just via the formula, sqrt(3200*0.5) = 40V.
However, 3200WRMS is quite a bit of power...hope ur subs and system's electrical are ready for it