Volti v Charney v Rethm

Not intending to throw a monkey wrench into your post, but you may also wish to consider Pure Audio Project Quintet15 with the center horn driver and four 15" woofers per side. I have the Trio (two 15" drivers per side) with horns, which may work for your room size (my room is smaller) but if you can spend more I would think the Quintets might better fill your room. Absolutely love the horn and open baffle combo sound of these. And at 96+ dB, very easy to drive. These have become my "forever" speakers. Good luck with your search!

I too love the Pure Audio Project Quintet 15 speakers.  I heard them with the center horn driver too, but, I would really love to hear them with the Voxativ fieldcoil driver-I have heard that driver in another system and it delivers a very vivid and lively sound.  Even with four 15" drivers, my concern would be bass energy in a large room because of cancellation of the front and back output which are out of phase (this reduces bass by 6 db/octave).

This is still an excellent recommendation that is consistent with the kind of sound that would be delivered by the likes of Charney, Rethm and Volti speakers.  An important reason for picking these kinds of speakers, to me, is that they can be driven by lower-powered tube amps which are my favorite kind of amp.  While a SET amp might not not be ideal, given the size of the room (the Charney might be efficient enough), there are many good lower-powered pushpull amps and a few OTL amps that would work well.  

I’ve only heard the Rethms at a few shows with unfamiliar associated equipment but they did sound beautiful in the midrange & very pleasing overall but not truly full range & not near as good up loud. They & I’ll guess the Charney’s, while being quite sensitive, are more akin to Quads in sound than to the Volti Rivals that I’ve owned & enjoyed for two years now. 
 

Rivals are very sensitive also & perhaps the absolute last word in gorgeous seductive midrange & fine etched, “hifi” detail some other even much more pricey speakers but can fill a big room like yours w/ live sounding, dynamic, full bodied & full range sound w/ ease. They are much akin to a high quality version of a big Klipsch, JBL or Altec horn loaded system. I too have a big room (23’x 25’x 12’ )  & the Rivals are a ton of fun  & if you like to really nice sound at any sound level but occasionally like to really open it up & wake up your hidden teen spirit ( as well as your wife or neighbors ( forget doing this in a condo)while maintaining excellent sound quality, then definitely check them out. 

... my concern would be bass energy in a large room because of cancellation of the front and back output which are out of phase (this reduces bass by 6 db/octave).

I believe this is true in general,  but as per Ze'ev, not so much with his PAPs as his drivers?/implementation? reduces this typical OB cancelation (exactly how I do not recall). Unless Larryi has measured this cancelation himself or knows a reliable study showing this for the PAPs specifically,  I would ask Ze'ev directly if this is a concern. 

I was impressed with the bass of both the Quintet and Trio speakers I heard because I like fast and nimble woofers that can keep up with the kinds of midrange/treble drivers they use which are also very fast and dynamic sounding.  I also saw that PAP employs the kind of woofers I like which use pleated paper surrounds—those tend to deliver very clean and articulate bass—but because they also limit excursion, they don’t deliver deep bass at high volume levels.  I admit that I don’t prioritize extremely deep bass, so I did not evaluate the PAP speakers I heard for that aspect of performance.  My “concern” was hypothetical and raised as something to look into rather than something I actually heard.

I did see that PAP did employ one approach to reducing back wave cancellation—they effectively widened the baffle with small side supports that extend backwards like the side of a conventional speaker with no back.  This is commonly done with open baffle designs.  It is generally considered better to “widen” the baffle with such wings than to use a wide front panel because of problems with reflections off a wide baffle and diffraction issues.

Aside from the particular demands here--large room and high ceilings—my favorite speakers mentioned here are the Charney Companions with AER drivers and the Quintets.  If corner placement works, I would add the fieldcoil version of the Audio Note AN-E to my personal favorite list.