No Fred, as Chris noted, there's distortion always present apart from actual clipping, which causes much higher THD numbers than "regular" distortion. As I mentioned above, actual clipping(not the rather strange usage of the term which some audio publications adopt)isn't related to a specific distortion number, but rather, occurs when the top of the waveform is flattened("clipped"), as viewed on an oscilloscope. Distortion at this point would be well above 1.0%, not the 0.3% or even 1.0% figure termed "clipping" by some publications.
Yes, blind tests have indicated that on music distortion isn't audible until it passes a threshold in the area of 1%. The more complex the music, the higher the threshold of audibility.
Maximum power ratings for all amplifiers for home use sold in the U.S.(and in practical effect, Canada as well)are governed by Federal Trade Commission regulations found in
Title 16, Part 432 . Basically, following a warm-up period, the amplifier must be driven at the full rated power for at least 5 continuous minutes, with the bandwidth and maximum distortion specified. Note that the number of channels to be driven simultaneously isn't specifically set, although the FTC interpretation has been to require at least two. The vast majority of receivers are rated with two channels driven, which is a more realistic measure in real-world home use than an all-channels driven measure adopted by some manufacturers, but which doesn't reflect a realistic scenario outside of the testing lab, with all channels simultaneously operating at full power for 5 minutes continuously.
The irony relating to published official power ratings is that this is the only area in audio which is governed by relatively strict regulations and where we can be confident of getting what's claimed, yet it's an area where many suspect "exaggeration" "inflation", etc. , which no manufacturer would risk. Some of those voicing such suspicions are also the same individuals who have no problem with believing in magical sound properties in various items, even pieces of wire, despite the lack of solid evidence.
