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)

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)