DUNLAVY SC-VI

A lot to consider. The room is being made out of an existing structure which is empty and stripped to open wall studs and open ceiling joists. The slab it sits on is within inches of 20 x 20. If I keep it 20 x 20 then I do not have to do any new roofing, supporting walls, etc.
Putting the speakers on the short wall makes sense to me acoustically since it makes the room more "amphitheater" like.
Kind of like a horn. The only deal, is that I wanted to also add a Runco and 120" screen for a... dare I say it... um... home theater... the 9 foot high wall would be ideal for a sreeen.... perhaps I could do an automatic drop down screen a little off the short wall, so I do not have to try to mount it on a 6 foot wall...

Not a bad idea putting the screen as a drop down.
If I were you I'd invest in a good acoustically transparent screen (so as not to cause adverse reflections from speakers) and hang it so that it's approximately even with the baffle of the speakers. If you use the short wall side this will put it out about 6ft. which will also raise it by ~1ft. which should be plenty high.

It all sounds interesting. Keep us posted!

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!

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.