This is not easily answered, nor is it cut and dry. I can say from experience, my 20x12x8 room gives me a very flat response.
Ideally, you wouldn't want any parallel walls, but unless one is building a room from the ground up, this is hard to find. Small room are worse than big rooms, and square rooms, or rooms that have similar dimensions that are close to square, are the worst.
The biggest problem with rooms is bass. Frequencies >300hz are easily managed with room treatment. Depending on the room, the dimensions can either help or harm the low frequency response. Room dimensions equal to 1/2 or 1/4 of the wavelength will be amplified by the room boundaries, for example, a 20'x15' room will have modes at 28hz and 37hz, 28hz being amplified by the length, and 37hz being amplified by the width. The distance of the sub from the wall, and the distance of the seating to either wall, will determine the amount of gain, for example, if you were to place the sub in the corner of this room, and seat yourself along the side wall, you would experience a large peak at around 40hz. Ideally, you would place the MLP at around 38% of the distance from the front wall or rear wall, which would achieve the flattest response.
So how can it help the low frequency response? Let's say you have a subwoofer that begins rolling off at 30hz, and front speakers that start rolling off at 60hz. Now let's say you place them in a 28'x15' room. At 28x15, the fundamental room modes are 20hz and 38hz. Assuming the room is closed, and the walls offer excellent rigidity, the response of your speakers should theoretically be extended to 40hz, and the sub down to 20hz. I say this from experience.
My front speakers have an anechoic -3dB point at 66hz, in my 20x12 room, this is extended to 50hz -3dB. I have a similar set if speakers in an 18x15 room, giving me a response down to 38hz -3dB.
How can it hurt? My bedroom is 12'x11', with modes at 46hz and 51hz. Since both room modes are close in frequency, this gives me a nasty 10dB boost at 50hz, making the bass sound boomy at one note.
Speaker and seating placement is equally important. Moving the subwoofer out and away from room boundaries will reduce the interaction at the room modes, whether or not this is ideal is entirely dependent on the room dimensions and subwoofer in question. Placing the couch directly against the back wall, and the sub in the corner of the front wall, will give a boost of 6-12dB at the room modes, which is likely to sound boomy. Directly in the middle of the room, both the reflected sound and direct sound meet with equal power, causing what I lime to call a "bass vaccum", at 38% from the front or rear wall, the level of the direct and reflected sound is more or less equal, leading to the flattest response.
As you can see, " ideal" dimension depend entirely on the speakers in question, and the seating arrangement. A pair of small satellite speakers that roll off at 80hz may sound great in my tiny square bedroom due to room gain, but a sun with a deep extension to 20hz will sound boomy. Small room and square rooms especially have problems with flutter echo at high frequencies, but like I said, this can be easily treated. To tame the room modes at frequencies below 100hz requires massive bass traps, since the absorbing material must be as thick as 1/4 wavelength to properly absorb it. It's impractical to place rigid fiberglass panels 6' out from the walls to tame a 50hz room mode. You're better off using multiple sub's on the middle of each wall and EQ to tame it.
Brick&concrete will help with room boundary gain, and also contain some of the sound in the room since it's far more rigid than drywall, but will also give very loud reflections at high frequencies. I would place drywall over the concrete, and then treat the walls accordingly. Place the listening position at 38% from the front/rear wall, and the speakers 22°-30° apart. Use diffusion at the first reflection points and absorption on the rear wall/ceiling.
I would also look into adding a center channel and surrounds. Even if your music is encoded in 2ch, Dolby surround or Prologic II does a fantastic job of pulling the ambiance out if classical. Stereo is flawed and can never properly recreate the 3D soundstage of classical music. Floyd Toole has spoken a lot about the shortcomings of it. Many "purists" refuse to use matrix upmixing, but Dolby surround does not add anything that isn't already in the music. When classical music is recorded in a large auditorium, the reflected sou d arrives at the microphone later than the direct, Dolby surround analyzes the signal in both the frequency and time domain, to properly reroute that reflected sound to the surrounds, where it should be. The lack if a center channel causes comb filtering, and requires the listener to sit directly in the sweet spot for the imaging trickery to work, with Dolby surround, the imaging stays regardless of seating position.
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