Yes I agree there is alot of factors involved and typically the consumer is mislead to believe something that is not. A 100watt amplifier should never be considered to produce 100watts for extended periods of time, to me this is logical. If you want to run 100watts continous then you need the head room to do so and should require doubling the power to 200watts.
The amplitude of an AC waveform is its height as depicted on a graph over time. An amplitude measurement can take the form of peak, peak-to-peak, average, or RMS quantity.
Peak amplitude is the height of an AC waveform as measured from the zero mark to the highest positive or lowest negative point on a graph. Also known as the crest amplitude of a wave.
Peak-to-peak amplitude is the total height of an AC waveform as measured from maximum positive to maximum negative peaks on a graph. Often abbreviated as "P-P".
Average amplitude is the mathematical "mean" of all a waveform's points over the period of one cycle. Technically, the average amplitude of any waveform with equal-area portions above and below the "zero" line on a graph is zero. However, as a practical measure of amplitude, a waveform's average value is often calculated as the mathematical mean of all the points' absolute values (taking all the negative values and considering them as positive). For a sine wave, the average value so calculated is approximately 0.637 of its peak value.
"RMS" stands for Root Mean Square, and is a way of expressing an AC quantity of voltage or current in terms functionally equivalent to DC. For example, 10 volts AC RMS is the amount of voltage that would produce the same amount of heat dissipation across a resistor of given value as a 10 volt DC power supply. Also known as the "equivalent" or "DC equivalent" value of an AC voltage or current. For a sine wave, the RMS value is approximately 0.707 of its peak value.
The crest factor of an AC waveform is the ratio of its peak (crest) to its RMS value.
The form factor of an AC waveform is the ratio of its peak (crest) value to its average value.
What is really interesting is the .707 factor is also a factor used in trig.
As long as RMS is calculated correctly it is a viable way of showing a true RMS watts. This is more accurate then what manufacturers try to get the consumer to believe.
Tell me what your procedure would be for checking true power output?
I have a friend that is a disc jockey and is totally nuts with checking a new amplifier. He goes through the whole procedure and if it didn't match up to it's specs it goes back.