DUNLAVY SC-VI

At first I was going to suggest the same thing as Unsound.
But I ended up not doing so for a couple reasons. The idea of having a sort of anteroom for the equipment would be a good thing but lead to other complications of access.

Ultimately I rejected the idea of making the room slightly smaller (in order to avoid parallel walls) was because of the sheer impact that the SC-6 has. If it were the SC4 or even the SC5 you could get away with this. But the SC6 will energize the room with so much low bass that it will be hard to avoid the rumble. You may alleviate some room modes higher in frequency but exacerbate the lower ones.

Of course at this point it is all guess work. But based on a good amount of experience with the SC6, I would avoid going any smaller in total room volume. FWIW.

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.

Now I am thinking about raising the roof 3 feet, which would give me a 12 foot ceiling on one end and a 9 on the other. I need to roof anyways so I could do this all at once. Am I nuts ?

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.

Raise the roof??
Yeah! Go for it if you can.

Unsound is right, the 20x20 thing will still be a problem but it won't be too difficult to overcome. Depending on how your ceiling ends up there will only be one major room mode and then a smaller one that will be offensive. There will be multiples beyond that but they won't be huge. The big ones can be alleviated with acoustical treament. (Helmholtz if you really want to get into it.)
But given the 20ft. dimension you SHOULD be able to find an optimum listening spot that crosses between a peak and a null of the modes.
Note: all identical room dimesions are bad. But the bigger the room, the more their issues get pushed lower in frequency. Thus, 20x20 isn't ideal but it's better than 10x10!