The actual performance of a loudspeaker design in terms of power handling is that they are designed to handle bursts of power for short timeframes. If you were to feed a speaker rated at 200 watts with a sound frequency (say a single 750hz tone) played at 200 watts for a sustained period, you would cause severe damage. That is what I believe Ian alluded to, that they could withstand that power level for music - which is usually made of short timestamps of certain frequencies at the levels mentioned.
Something to keep in mind regarding amplifiers and volume controls - they are not designed as linear devives. This means that Amp A can have a rating of 20wpc and stop delivering power and start clipping when the control is turned to 4o'clock, and Amp B rated at 100wpc can reach this point with the control at 10o'clock.
Your NAD is of solid design, and I would say it is entering clipping and gone beyond producing clean sound at the points you mention. It will react differently to different frequencies it is asked to amplify. Even with 'soft clipping" enabled, the amp WILL clip, as this does not stop that phenomena.
