DUNLAVY SC-VI

I think most here would agree that the SCVI's would be overkill in this application. Tapering the side walls to create a back wall of about 13' would be just about perfect for SCIVa's. Besides the added construction challenges, the effort would create a real concern if he wanted to sell the house later. But, oh, what a room!
Could be close to audio nirvana.

What do you mean by a room build?

Alissatweaks, That's what I thought. With that in mind, is there a reason the room is 20' X 20'? Doubling dimensions is usually not the best way to go. I don't share your disdain for audio processors, and with those dimensions, such a piece of equipment might be helpful. I agree with Daverz that firing the speakers from the short ceiling side might be preferable, with that in mind the Dunlavy SC VI's height might be an issue.

Have you considered reducing the size of your room? You could have better dimensions, a separate area for your gear that would be exposed to less vibrations, and permit easy access. If your really got creative you could avoid any parallel walls. That would probably sound really great!

Raising the roof, might be a good idea. Changing the 20 X 20 dimensions would be a better one. There are classic ratio models that I strongly suggest you consider before proceeding. One models suggests "...that the second largest dimension should be 1.25 times the smallest, and the largest should be 1.6 times the smallest." The Cardas web site had some interesting recommendations, including one for rooms without parallel surfaces. Bear in mind, many recommendations might be based upon the idea of placing the speakers behind the short wall, the opposite of what Dunlavy recommended for his speakers. Though I seem to be in the minority here, my experience has shown the Dunlavy recommendations to be spot on. The one caveat is that one should not ignore the recommendation for room treatment directly behind the listener. IMHO, consider it mandatory. Though a smaller room might not technically have complete frequency response, with room lift, you'll probably still have satisfactory bass response. Overall sound will probably be better than overblown lumpy bass due to poor dimensions in a larger room. While your at you might want to considering dedicated power lines. Good luck.