Volt Amps (VA) is commonly used in rating power transformers. There is a reason for that. But that is typically at 60 hz, so one variable is made non-variable. VA is not a suitable power rating for a power amplifier, since that is not the job of an amp. Unless, of course, you want to use your power amp for a welding power supply. How about rating speaker power handling by VA? Would be just as suitable, right? Wrong. For many of the same reasons.
Having said all that, if one insists on using the VA method, then AT LEAST use a signal source of one frequency from a CD or USB source. THEN clamp on the ammeter and use a VOM. THEN you can standardize your power output to a known standard.
Fact is, I used to do that years ago when building/testing my own amps. Not fullproof, but it does give a measurement to set standards by, for power output only. It in no way tells you how your amp SOUNDS.