Harman Kardon rates receivers as: all channels driven simultaneously, 20hz-20khz, at 8 ohms, to .07% thd.
NAD is, by far, the most demanding with ratings. All channels driven simultaneously, 20hz-20khz, at 4 and 8 ohms, to .08% thd.
The reason rating with all channels driven simultaneously is important is this: It gives a good representation of how large and adequate of a power supply the manufacturer chose to use, reguardless of whether or not is was 20hz-20khz or 1khz. Although, 20hz-20khz would be more demanding. Basically, it can show whether the manufacturer "chose to wear the bare minimum amount of flair or go above and beyond and wear extra
." (Use the smaller power supply to just acheive the FTC rating or use a larger one capable of meeting stricter standards.)
I am sure no one listens to just sine waves in the comfort of their own home.
In the case of receivers going into protection all channels driving simultaneously I would say that the manufacturer put it in as a safety guard for the smallish power supply to protect it from over current as that is it's design. Granted all amplifiers need a protection circuit, but when I see protection kicking in BEFORE clipping (all channels driven simultaneously) at resonable distortion levels, it makes one question the integrity of the design, and how truthful the manufacturer is.
As for a rating standard from the FTC, we should have one for surround receivers. It should go some thing like this: ALL channels driven simultaneously, 20hz-20khz, at 8 ohms to <.1% thd. for at least 3 seconds. Chances are, however, this would not happen, as it would be met with much backlash from companies thriving off of the current rating system. Why? Because, for one, their manufacturing costs would increase as now their have to give you (the customer) a more powerful amplifier in order for it to stand up to the new standard.
I do not see why people do not embrace this as consumers. In the end, it is you and I, the consumer who wins. We get more powerful amplifiers at a cheaper price. Rather than the current trend of weaker amplifiers for the same old price. You can choose to support the current rating standard and get less power for your dollar, or push for a stronger one and get more. As for me I choose the latter.