Why "splay" walls 4 custom listening room design?

As an Architect of high end residential, I've had quite a few opportunities to design/ build and research the room issues. Two years ago my client spent quite a bit of money ($15,000) having an acoustics expert calculate his $200,000 home theater needs. This guy designs home theaters, small scale preforming arts facilities, movie houses and churches.
In home audio our needs are unique in that the scale is very small, reverberation rates, sound enhancement and echo are nominal issues. This could possibly be what Everest is discussing. Our issues are mostly side wall and floor/ ceiling first order reflections (slap echo) and frequency exaggeration.
A very small splay (narrower at the front) will alleviate most of these frequency nodes, but some thought could be put to the amount of splay. Too much could reinforce the projection and act as a "horn" and not enough will not achieve the intended goal. Ceiling and floor sloping can be more dangerous, in that the sound angles off a sloped ceiling (in the same fashion as the walls) could end up directing the reflection bach to the listener (especially with multiple seating locations). Think of a dome, it reinforces all sound to the middle, very bad in our situation. A slope down the axis of the room however could disperse the reflections very well.
The consultant we used had a very elaborate computer program that he was able to model the full spectrum of possibilities for the best solution. He was able to produce an audio track for any position in the room to determine what every seat would hear. There are cheaper programs available that could do some of the same things, just not quite as involved.
The rule of thumb I would use is the ratios spelled out in Everest's books or Robert Harley's book to determine the best ratio and size. If you did splay the side walls, I would use the width recommended as the average (mid point) of the splayed wall. I have been using a 2 ½ degree angle on my designs with very good results. The ceiling I would vault if possible.
As to how important this is, it completely depends on how far your willing to go. If your intending to get every ounce from your system with power conditioning, cables and tweaks, then this makes sense. If your happy with 80% of perfection, skip it, it's not worth the hassle. If you do however pursue this I would recommend you look into what wall materials your using. One layer of drywall over wood studs can give a soggy base response compared to a more rigid construction. You might consider building in a true base trap in the design, what acoustic treatments are you willing to live with, if non then these issues become extremely important.
"Acoustic treatments are a band-aid for problems not addressed during design, not the answer" according to this expert.

Clueless, I too remain clueless as to who (if anyone) actually knows the facts. Here are some random thoughts based on my personal observations and quite a bit of research and continuing education.
Buildings react to the low base signals in what I would call an organic way. Think of how wood responds in instruments and how tweakers have used woods of differing densities to "tune" there rooms. The material itself actually interacts with the sound wave and can take on it's characteristics for that moment. In the case of gypsum board (gyp. bd.) the material is only ½" thick in normal construction spanning 14½" of air over the height of the wall. This is in effect a long drum with a loose skin. The more screws used to fasten the gyp. to the studs may tighten the drum skin some, potentially changing the frequency but the issue remains of a thin skin over a wide space. I would assume more studs, 8" on center would alter the bass response quite a bit, although I've never tried it. The "experts" I've worked with suggest 4 layers of ½" gypsum board (each glued and screwed) as a minimum construction in a church, play house or theater. This is a minimum, they prefer 8" concrete block or poured concrete. The concrete has very little reaction by nature with the bass frequency thus allowing it to pass through. Base waves do not bounce and reverberate off materials like higher frequencies do. If you have ever been late for a concert in a stadium or hall built from concrete, you will know your late because the base is heard from the street. The problem for us with concrete is it reflects the high frequencies, but not a lot worse than gyp. bd. The paper on the surface of gyp. bd will tone some frequencies in a slap echo scenario. For most of us, concrete is not practical unless your listening space is in a basement. Then I would try to make sure both side walls were made of concrete. In an above ground situation with wood studs, the only practical solution is thicker gyp. bd. In the four layer scenario the net result is 2" of gypsum, in that it's glued to each preceding layer it acts as a solid 2" gypsum shell. Gypsum is a softer and a more absorbent material than concrete, but the two are very similar as a basic structure. I'm thinking the gyp may have some properties beneficial or possible a hindrance to the low midrange and upper bass where it may absorb some energy. Would this be released at a later time causing smear? Does a thicker stud wall with insulation help defuse this energy along with base signals? It's also possible to retrofit an existing room by adding three layers of gyp, the problem here is all the trim must be removes and the door and window frames would need to be extended.
Anyway, the idea as I understand it is to make the walls rigid but not created a drum. Once the space is created that has the least interaction to the base signal, we can consider wall materials. Much has been written on the wall coverings. I have used fabric pleated curtains, ceiling to floor on home theater designs. This is much like a movie theater, I think this is an attractive solution that responds to the greatest amount of frequencies due to the pleats. The added benefit of curtains rather than something applied to the wall is it's ability to double it's potential by absorbing not only first order but reflected waves.