Sdcampbell, nice post. I might add that the impedence load is also an important part of the rating. "150 watts per channel, both channels driven, 20-20kHz, at 'x' percent distortion into 8 ohms" would be a more complete description (add RMS, which averages the power across the sine wave, into it and away we go). In the old days, during the power rating wars, "peak power" has also meant the power rating at the top or peak of the sine wave. RMS power is about 70% of that. "Peak to peak" power (and I wonder if that isn't part of the PMPO abbreviation) is double peak -- i.e., it counts the absolute value of the power to the peak in both the positive and negative sine wave cycles. So, peak to peak may be 200w and peak is half that at 100w and RMS is about 70% of that at 70w -- all are accurate descriptions of the same amp, but they measure power in different ways.
Getting back to the impedence load, a 70w amp into 8 ohms might deliver 140w into 4 ohms (if it has the power supply and design capacity to do so) and 280w into 2 ohms, which sometimes is the impedence for very low bass. So, it is perfectly legitimate to describe the same high quality amp as 70 watts RMS into 8 ohms and 600 watts peak to peak into 2 ohms. Add in all of Sdcampbell's other mentioned caveats (e.g., distortion level differences, applicable bandwidth) and you could claim even more power from that amp. Good thing this hobby's so simple ;-)
Getting back to the impedence load, a 70w amp into 8 ohms might deliver 140w into 4 ohms (if it has the power supply and design capacity to do so) and 280w into 2 ohms, which sometimes is the impedence for very low bass. So, it is perfectly legitimate to describe the same high quality amp as 70 watts RMS into 8 ohms and 600 watts peak to peak into 2 ohms. Add in all of Sdcampbell's other mentioned caveats (e.g., distortion level differences, applicable bandwidth) and you could claim even more power from that amp. Good thing this hobby's so simple ;-)