Mojo, Good suggestion. Measuring the voltage and the current would give a reasonably close measurement.

The only variable not accounted for is distortion. I would recommend that you use a Piezo tweeter wired across the speaker terminals of the amp. Play a tone that is well below the frequecy response of the tweeter (like 100hz). Turn up the volume until you hear a buzzing in the tweeter. The buzzing you hear is the harmonics generated by the amplifier clipping and squaring off. Then you could take your measurements and run the math.