Amp output levels

If its me, I will use a dummy load, say 8 ohms and 50W or more(measure both resistance accurately before connecting). Play a test track, say 1 Khz tone at a desired vol pot level. Measure the voltage across the load. Calculate the current. Calculate the power.

P=EI x power factor angle
If I want to just see a discrepancy, I can set Power Factor to 1 and use simple equations like:

P=EI
P=V2/R :(V squared over R)
P=I2*R : (I squared * R)

Measuring each amp output and calculating the difference is a poor method. Rather, with a Mono signal, measure across the Left and Right amp outputs. That is a direct measurement of the difference.

Measuring each amp output and calculating the difference is a poor method.

Not if you are measuring with an Oscilloscope for voltage or a power meter for power with a reference test tone!

Could you please educate me a "better" method to measure power difference (rms only not VAR or VA)?

Amandarae...Your way will involve comparing measurements of two relatively large voltages. Even a small percentage measurement error of the large voltage will imply a large difference.

If you measure the difference (across the two hot amp output terminals) the voltage will be small (ideally zero) and measurement error will not be significant. (You will be on a low scale of your meter). If you want to express the difference in terms of dB relative to the output signal you will need to also make a measurement of the output (either channel).

By the way, an Oscilloscope is a useful tool, but not suitable for accurate voltage measurements, unless, perhaps, it is a digital scope that has a numeric display of peak voltage (in which case it is really a meter, not a scope).