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).
