Isn't science wonderful? When someone at work gives me their opinion, no matter how sensible and intuitive it may seem, I ask for data. The data always brings new learnings.
Regarding Ian's test, I and others who have listened to my systems, have found that we fatigue easily and object when the highs are distorted. I don't know what level of distortion this represents because I can't measure it but it's always the highs that are real nasty. I also don't know if this is THD or inter-modulation distortion.
Having said all this about the highs, distortion in the lows is nasty in a different way. Although it doesn't fatigue, it negatively affects imaging and soundstage and muddies the sound.