Has anyone upgraded from an AudioTechnica ART9?

I strongly doubt that "many thousands" of the very expensive higher power Pass amplifiers are sold, if you're speaking of a single model.  If you take total production over the last years, maybe.  Otherwise, I agree.
Johnss, No one I know has ever heard the ART1000.  Have you?

@audiofun uses an ART1000

Magico is a typical modern day speaker. It loves to be driven by quality high power solid state amps. I have heard some of the best installations of Magico and it always shone through with a nice and proper 500 watt amp.

Dear @pani  : Speaker efficiency/sensitivity of 91db like the Magico I listened means that you can get 91db SPL at 1 meter with 1watt and 91db is high volume for any one.

Speacker impedance curve is other parameter to take in count but as I posted I did not be aware of any problem where  I can tell the amp was in " trouble " handling the Magico's.

R.

Yes @rauliruegas I understand the sensitivity and impedance curve properties and their effect on the output. An amp not being is trouble is not enough. Typical modern multi-driver speakers like Magico, Rockport, Avalon, Focal all need a lot of current to come alive and show their full frequency response with uncompressed dynamics. Firstwatt amps will never push them that way because they do not deliver such high currents. The XA has large array of transistors at the output which allows large current swings. It is just a simple comparison between a small amp with a small power supply and a big muscle amp with a large power supply.