Mcintosh 6900/Klipsh Cornwall III - a good match?

You seem determined to place a full range, high volume speaker in a smallest square dimensioned room. It's a combination that will not mesh very well.

simple and superb.

Cornwalls can fill a concert hall but will choke in a smaller room. Go for a pair of Hereseys instead. They have almost the same sound quality, just smaller. Klipsch don't seem as dependent on the wall placement as others, probably due to the massive cabinets.

No way I'd do that combination. First while the 6900 sounds ok but a little harsh on the highs, putting it through the horns will not produce a plesant listening experience. It's pretty hard to get any modern SS amp to sound good through horns. The Mc 6900 doesn't even sound that great through Focals and JMLabs speakers I've heard it through. There are at least 5 or 10 other SS amps I'd choose over the McIntosh. Now if you upgrade further to Mc's high end stuff or tubes you'll be doing better.
regards, David