DUNLAVY SC-VI

I, similarly to Pryso's friend, had Duntech Princesses in a 20 x 18 room with openings going out from the back side so it presented a larger acoustic space, and I think that was the most a room of that size could handle (I tried the Duntech Sovereigns, equivalent of the Dunlavy SC Vs, and felt they were just a little too large for the room). I think the room size you are talking about might be better suited to the SC IV or possibly the V. Part of it is the distance you need to sit back from the speakers for proper driver integration as Pryso notes, the other is bass loading in the room. And I agree with others above, try to avoid a square room if you can, since it is being built. The VIs are a great speaker system, but I think to hear them at their best you might need a slightly larger room or some room equalization to tame the bottom octaves.

20x20 is a bad idea.

Here is some helpful info: http://www.cardas.com/content.php?area=insights&content_id=41&pagestring=Listening+Room+Dimensions

OK, sounds like some options may still be open here so I'll jump in one more time.

Since you are/will be building the room, why square? If you are locked in to a foundation already constructed, look for ways to open the new room into an existing space to change the standing waves. Otherwise, if not too late, redesign the room.

Speaker choice -- I owned Duntech Princess speakers for 19 years and remain a big fan of John Dunlavy's designs. However, I never would have traded my Princess even up for a pair of Duntech Sovereigns for my 20' x 17' x 11' room. It is simply not large enough. I heard the DAL SC-VI only once, at the CES. But I also heard the Sovereigns in a multiple of rooms of various sizes. I would consider a room about 28' x 19' x 11' to be the minimum, AND I think the SC-VI may need something even larger to perform near their capability. With a room anywhere close to your stated 20 x 20 size I would look for Duntech Princess or DAL SC IV or IVA.

Good luck.

The SC6 was intended for rooms no smaller than 20' x 30'.
The size of the speaker (mostly the woofers) are there to be able to reasonably and accurately fill a space this big.
If you try to fit them into smaller spaces you not only run out of room to be able to properly adjust them but you end up having to listen at less than ideal volumes so as not to overload the room with bass.

That being said, if you were to use them you would want to put them at the shallow end with regards to the ceiling. Assuming exact dimensions stated for the ceiling would put the back of the speakers no closer than 3.5ft to the back wall. This puts the baffle a little over 6ft. from the back wall. And assuming an exact listening distance of 10ft. leaves you with just about 4ft. from the listening position to the back wall. All in all, these are not bad dimensions to work with. The distance behind the speakers should give you a nice deep soundstage. And the four feet behind the listening position will alleviate a good amount of that reflected distortion.
Having the ceiling slope upwards as you go towards the listening position will shift the ceilings first reflection higher and pretty much send it overhead of the listening height. Therefor, the first place you want to add room treatment is on the wall behind the listening position and have it extend to at least 8ft. high.
Some diffraction treatment on both side wall first reflections would be the next priority.

You wont have any room left over to control the bass but this setting should work out pretty well for the rest of the spectrum.

In the manual for the SC-III, Dunlavy recommends sitting about a foot from the back wall and treating the back wall with 4" thick acoustic foam (four 4'x2'x4" panels would probably do it). Or at least that's the recommendation for a room about 20x13 in size, where a 10' listening distance would put you near the back wall. (I still have my SC-IIIs and wish I had a room this size to stick them in.) It would be interesting to see what the manual for the SC-VI recommends.