Big, big room -- which 10-20k speakers?

What terrific feedback! Thanks so much everyone for your help with this.

In answer to a few of the questions asked above:
- I have fairly eclectic musical tastes. I listen to old folk and country (think Bascomb Lamar Lunsford and Ralph Stanley), jazz (Jarrett, Monk), rock (Pixies, Patti Smith, M.I.A, the Dead.), symphonic classical, opera, world (mainly Japanese since I lived there for so long, and Brazilian of the quasi-jazzlike Milton Nascimiento variety). I really would not want to have my system be a major impediment to listening to any type of music

- My normal listening position will be about 12-15 feet from the front wall (which is actually windows) and slightly off-center. We will also use the room for entertaining a few times a month so being able to fill the room with low to moderate volume music would be useful

I am totally unfamiliar with horns. Would something like Avantgarde Trios with subwoofers be a good approach? I'm a bit concerned about the bass module crossover, and soundstaging

Thanks again for the advice! I've already spent hours searching the web trying to form a view on everyone's recommendations.

Man, I'd love a huge room. As an earlier poster suggested, you should try
to take advantage of the benefits. I don't like overdamped, dead sounding
rooms. And the fact that the ceiling is that high may help if you could
suspend some appropriate diffusion- almost like open wood frame 'cells' in
a matrix (think honeycomb, just not necessary that shape of cell) or slats.
The
same width/length may be problematic, but that could be rectified by
erecting something that functions as a freestanding partial wall behind you,
perhaps even partly used as vinyl or media storage if you were inclined. Or
flip the set up around, so that fake 'back' wall and the front wall are the
sides. See below re the glass windows issue, which could then be the front
and back walls if properly treated. In effect, you could create an acoustic
environment within the larger room.
There are good people who know acoustics, know how to measure, but I'd
be reluctant to let someone loose on heavily treating that room until you
know what you have and how to take best advantage of it. What are the
walls, ceilings and floors made of? You'll obviously have to deal with all the
glass, but the right window treatment (and perhaps not even 'acoustically
endorsed' but along the lines of those heavy curtains used in theatres)
could work. Too bad there's no way to show us a picture or three, unless
you set up a system page and post the pics there. I'd love to see the room.
I'm not going to advocate any particular equipment to you. The best
sounding room i had so far, not nearly as large, was in an old brownstone,
very high ceilings, real heavy plaster walls, very wide old plank flooring.
The room sounded marvelous on its own with minimal treatment.

the trios will fill your space with ease but they are beyond the budget mentioned. Depending on required volume levels you can add subs. the new 231 subs ( 2 x 12" driver per side) sound very good and I guess you would at least need 2 subs her side. this is all way beyond the mention budget but would be close to perfect to fill that huge room.

Maybe consider the large PBN whip ass speaker (WAS)

Almarg makes good sense as usual :)

The larger Tannoys are another good possibility.