I was going to do a longish post on distortion and ATCs, including some online references, but that needs more time than I can spare right now. So, let me contribute to this thread very briefly:
In conventional drive units such as those used by ATC, non-linear distortion tends to be directional, i.e., the distortion is probably worst precisely where HFN measured them, directly in front of the drive units.
Btw "under 1%" can mean anything between .0001 and .99%; it probably varies with frequency. If the distortion is under 1% even in the crossover region, it is extremely low, most passive systems will have much higher distortion there-
In contrast to the controlled-dispersion monitors that Thorsten prefers, I understand ATCs will probably be listened to off-axis. The studio installation pictures on ATC's website certainly point in that direction. The non-linear distortion will be substantially lower under these conditions.
I don't really know how the ATCs will compare to Thorsten's preferred alternatives under those conditions. Maybe DominicT's measurements will provide a clue.
Subjectively, I find that ATCs have quite low distortion. The Quads sound cleaner, of course, but don't have anywhere near the oomph. The ATCs sound better than any number of "domestic" speakers I have heard.
I would certainly like to listen to the Geithains.