To accurately measure the power output of an amplifier, all you need is a true RMS voltmeter with sufficient bandwidth and some 8 ohm power resistors of appropriate wattage rating. For a sine wave input/output, it's much easier as any RMS meter will do. Voltage squared divided by the resistance is power.

To be more accurate you could use an oscilloscope to view the waveform. This would allow you to adjust the drive until the amplifier was just below clipping. The oscilloscope will give you the peak value of the output waveform. For a sine wave, the RMS value is the peak value divided by the square root of two (about .7071). Apply the formula above and you have an answer.

McIntosh used to run clinics at various dealers where you could bring in a preamp or power amp and they would measure it and give you harmonic and IM distortion figures as well as the power output of the amplifier (at no charge). Most good repair shops have instrumentation that can measure amplifier output and harmonic distortion. For a fee, they'd probably be happy to measure and give you the numbers.

Steve