Hello everybody, I am considering a pair of SC-VI for a room build, and the room is 20'x20' with 9 foot ceiling on one end and a 6 foot at the back wall. I would like to get some feedback as to the size of the room for these speakers. Any responses would be appreciated..

Thanks !
Unlike the usual equilateral arrangement, a typical Dunlavy arrangement would be to start with the speakers about 11 feet apart and the distance from listening position to each speaker of 10 feet (68 degree angle between the speakers). You might be able to get the speakers 12-14 feet apart in that room. The soundstage should be fabulous.
What do you mean by a room build?
A good friend had Duntech Princess (more similar to DAL SC-IV) in a 20x20 room with a 9' vaulted ceiling and they sounded fantastic.

HOWEVER, read any source on acoustic design and they all recommend against a square room (avoid standing wave reinforcement). I think my friend's room worked so well only because one end has a 6' wide opening into a smaller room plus an open door way leading to another room; in other words his room was not square acoustically.

I would suggest contacting Rives Audio or other acoustical designers to discuss your situation. And I will suggest that all Dunlavy designed speakers work best with placement well out from both the front and sidewalls. I would be more concerned with sidewall distance than distance between speakers (meaning 4-4.5' each side with 10-12' between speakers [centerline to centerline] in a 20' room). In my experience the 10-11 feet to the listening chair that Daverz suggested will be the minimal distance. Such tall speakers need distance for the drivers to converge, particularly time/phase coherent designs like the DAL.
Room build means that I am purpose building a room for audio listening. Everything will be acoustically optimized including room treatments. Audio processors make me cringe even though they can be very effective. I think I have the width thing down pat, I am more concerned about low ceilings. 9 foot at the front wall and 6 foot at the back. 7.5 foot ceiling height 10 feet from the front wall. This means if they are 2 feet from the front wall, listening position should have a 7 foot ceiling height roughly. Perhaps I should place the speakers at the short wall facing the other way ?
I think you want the speakers firing away from the short ceiling side so that reflections are not "focused" on the listener by the ceiling, but I don't have any empirical acoustical data to back that up.
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.
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.
@ Daverz, you are correct in linking that recommendation with the room size.
Dunlavy made seating recommendations based on a list of priorities.
The first was to always use the long wall if possible. This was to first increase the space between speakers but, just as importantly, to increase the relative distance of the direct sound versus the first reflected sound from the side walls.

The second priority was to keep the speakers off the wall behind them as much as possible. This also was to keep the difference in timing between direct sound and reflected sound relatively high. It also helps to increase sound depth.

Of course, all Dunlavy speakers were designed for a 10 ft. listening distance. So the distance from the listener to the wall behind him/her was the last priority. And in normal size listening rooms usually meant the listener was right on the wall. In this case, the easy fix is to cover the area with thick absorptive material. Leaving this wall for the primary wall treatment also meant that you could get away with a lot less material since the closer you sit to the wall the smaller the area of treatment necessary.

Given these priorities it is easy to see how the size of the room will ultimately impact the placement recommendations. Hope that helps!
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.
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!

How did you end up with your room?

Can you send a picture?

I also own a pair of Dunlavy SC-VI. Room dimensions and positioning are super important, as already discussed.

I have not comment on your question. However, if you decide not to go with the SC-VI, would you be willing to share where they are available? I'd be interested in a pair at the right price.

Listen to UNSOUND. Get your room dimensions correct. I've owned SCIVs and SCVs. SCVIs do require a large room and I do think placing them on the long wall is best. I would look into narrowing one dimension of your room. Something like 20x17x 9. Dunlavys can be placed quite near the rear wall, so you might have enough room. If this doesn't work- consider a pair of SCvs.