watts = amps x volts is the equation I assume you are referring to. But, this is a general equation that assumes you know two of the three. Unfortunately however, if you didnt factor in the impedance the amp is running at, you cant know amps. Most amplifiers' output is directly related to the impedance it sees. If it puts out 500 watts at 4ohms, it will put out (approx) 1000 watts at 2ohms. For example, not referencing your amplifier.
One last but.... but if you measured input amperage at the amplifier using a DMM, you already know that without having to worry about speaker impedance.