I think the more speakers is for more volume (as long as the source can power it. Especially if they're hooked in parallel) I don't think there's an issue with phasing/smearing in a 20x20 room but I could be wrong. I suppose it has to do with the acoustics of the room too. I am sure all the cables will be within a few feet of length with each other too.
Ok....this is a common misconception. ANY time you have more than one source point for directed energy, if the source points are NOT aligned on the same axis, you will have time smear, ring modulation, amplitude modulation, freq. modulation, dead spots, "loud" spots, early reflections, etc.....all the bad things you don't want to have heappening to sound (Matty hit this right on the head). ESPECIALLY if you are trying to create a quasi-quad setup (5/6/7.1 does not apply due to the delay that the amp adds to reconstruct "room acoustics"). Quad is a BITCH to configure. Think about this.....in a quad system, you have 4 source points. These 4 source points have to be ABSOLUTELY lined on the X-Y vector axis, and ABSOLUTELY square. Even a misalignment of say....1/4" on any of the points (the tolerances for proper acoustical reproduction are SO miniscule its silly....1/4" is a MASSIVE ammount) will result in any of the above mentioned sound colorations......
If you've ever seen the inside of a major studio control room, have you ever noticed the monitors?.......ever wonder why they are always laying on their side?......ever closely examine a studio monitor and notice that the drivers are as close to being absolutely flush as possible?....There's a reason they are set up like this.....it's to avoid these issues. There are ways to treat a room if you can't set up a perfect setup. Ok, I'm beginning to ramble......back to what I was saying......
Having more speakers does NOT equal a louder system. This is a common misconception. If you have 100watts, that's all you got. Period. You can put 100watts into 200 speakers, and it will not play any louder than it would if it came from one. As a matter of fact, it would prolly HURT the performance more than anything due to the lovely rules stated by Ohms Law.....Adding more resistance requires more power to drive the speaker to the same db level output......this is basic speaker wiring..... 4ohm + 4ohm can either equal 2ohms (in parallel) or 16ohms (in series).......The lower that load is, the higher the current can be driving the load.
In other words.....to get more decibles, you need more current. The more current you have available, the harder you can drive a speaker, and thus get more db's out of it.....and of course, you all know that getting a +3db gain in output requires DOUBLE the power.....
dB (SPL) Source
194 Theoretical limit for a sound wave at 1 atmosphere environmental pressure
180 Rocket engine at 30 meters
171 Current SPL record for a IASCA Competition
120 Threshold of pain
80 Vacuum cleaner at 1 m
50 Quiet restaurant inside
0 Threshold of human hearing
* It takes an amplifier twice the power to produce an increase of +3 dB.
* It usually takes about 2-3 dB for a human to perceive a difference in a sound level.
* It takes +10 dB for a human to percieve a doubling of the sound level.
Smeared: Lacking detail. Poor transient response, too much leakage between microphones. Poorly focused images.
Muddy: Not clear. Weak harmonics, smeared time response, I.M. distortion.
That is all...