What are the important factors at work here? They are many, and they interrelate in complex ways. What size is the room? The problems presented by a room 11' by 15' are very different from those in an 18' by 26' space. Where are the windows and what type of window covering has been chosen? How high is the ceiling; is it flat or sloped? Is the floor fully carpeted? Is the room a dedicated listening space or must it, as is usually the case, be used for other purposes?
Designing the "ideal room" from scratch is usually the best approach. JGH's article in Vol.13 No.4 dealt heavily with room shape and construction (including the vitally important topic of standing waves). But building a listening room from the ground up is seldom possible for most of us. Fortunately, there is usually a great deal that can be done to make the best of an existing situation. True, short of major and expensive remodeling (which might turn out disappointing, in any case), you're generally stuck with a basic room shape and configuration. But beyond that, you can improve matters, or at least do much to minimize the negatives.
Acoustics is at least as much art as science, and offering specific advice is more than a bit dicey. Even the experts don't entirely agree on the recipe for the "best" listening room. But I won't let that stop me. Just remember that you'll be able to find knowledgeable individuals who will not agree with everything in this article, and in a particular circumstance they may very well be correct.
For reproducing music I personally prefer a room that is slightly too dead acoustically to one that is too live. How can you determine this without instruments? The room should be, first of all, comfortable to converse in. Do voices sound smothered and overdamped, echoey, or just about right? When you clap your hands sharply in the room, does it sound "alive" enough, yet without a zippy "aftertaste" that takes a noticeable period of time to die out? In home acoustics, a second is an eternity, a half-second a lifetime. This "ringy" quality is known as "flutter echo," and if clearly audible can defeat the best efforts to produce decent sound. Flutter echo is due to the successive reflection of (generally) higher frequencies back and forth between flat, untreated, opposing walls. It may be cured either by damping those wall surfaces or by doing something to break up their flat surfaces---making the reflections less directional by dispersing them. The best solution is to provide the room with a reasonable balance of dispersion and absorption. Wall hangings, pictures, bookshelves, and drapes and floor coverings of modest weights, are usually pluses; large areas of uncovered windows, bare floors, and totally bare walls are seldom desirable. Nor are their opposites---an abundance of overstuffed furniture, heavy drapes, and a heavy, thick carpet and pad.
Room symmetry is generally a plus. The environment to the rear and sides of each loudspeaker, especially, should be as well-matched as possible with respect to absorptive and reflective qualities. JGH had a number of important things to say about this topic in his article. There has been, unfortunately, a lot of otherwise well-intentioned advice bandied about in other quarters which can defeat the concept of symmetry. The most common is a generalization: opposing surfaces should differ in their absorption characteristics. There is a kernel of truth in this, but taken as gospel it can destroy symmetry. The concept seems to have originated in the days when no one was concerned with stereo soundstaging. In actuality, the left and right side walls, as you face the loudspeakers, should be as symmetrically damped, and as physically symmetrical as possible in the area around the loudspeakers---as should the wall areas immediately behind them. To consider an extreme example of a poor situation, let's say you have your loudspeakers on the short wall, each 3' from the closest side wall. If one of those walls has bookcases against it, and the other a large, undraped patio door, you may have satisfied the "requirement" of differing opposing walls, but you'll likely get a rather poor overall balance and stereo imaging. To a lesser extent, the left-right environment around the listening area is important. A very dead wall on one side and a very live one on the other will create an odd auditory sensation that something "isn't right," the severity of the situation depending on the proximity of those walls to your listening chair.
Symmetry of the floor and ceiling surfaces is less important and difficult to control in any case. Here, dissimilar opposing surfaces are the norm, with full carpeting on the floor and a generally reflective ceiling. The absorption characteristics of a wall-to-wall carpet and pad are less than ideal. It is concentrated mainly at upper-mid and high frequencies, though its specific characteristics vary significantly with different types of carpet and pad. Use of area rugs in place of wall-to-wall carpet has been proposed by some as a possible, less drastically absorptive alternative. It might work, though experimentation would prove costly. I prefer a modest-weight, wall-to-wall carpet and pad (footnote 1) if for no other reason than I suspect (without the statistics to back it up, I should add) that most loudspeaker designers do their final listening tests and "voicing" in fully carpeted rooms. Symmetry again, although this time of a different nature.
Footnote 1: Some experts hold that a thick, foam-rubber pad is worse than a natural-fiber pad because it tends to absorb more selectively: ie, while some frequencies will be significantly damped, others won't be at all. A natural-fiber pad is said to have an absorptive character that is more even with frequency.---JA