Dual Subwoofer Placement

Its not a matter of belief. Its a matter of physics.
The more sub locations the more bass modes and the smoother the bass response.

The more subs the less volume each one has to put out and so again the smaller the modes and the smoother the bass response.
This is all due to the fact low frequency waves are very long. None of this changes when the room gets smaller.
The problem is people don't understand what's going on. They buy a sub to hear low bass. So they turn it up enough to hear low bass. This excites room modes making boomy bass. They conclude its the room. Or the sub. Or anything but what it is, the universal physics of long waves in small rooms. The solution is many small subs.
Its not that 2 or 4 is best. Its that more is better.

Asymetrical placement of the subs helps control the very worst room modes.

jt35:
"agree there is no rule of thumb on this one...I'm getting great results with one sub, inside and slightly behind right speaker...friend with similar system gets best results with 2 slightly smaller subs, on outside...many believe 2 or 4 subs are best, but not in every room or every system..."

jt35,
     I don't agree, there is a rule of thumb on this subject of in-room bass response in smaller domestic rooms that's been researched scientifically by acoustical experts, Dr. Earl Geddes and Dr. Floyd Toole.  Both have documented their research, methods, results and conclusions in published White Papers that you can google and read completely on-line.     Numerous other acoustic scientists have independently repeated their experiments and verified their results.  I've read both of their White Papers and, if their conclusions were to be summarized in a rule of thumb guideline for in-room bass response in smaller domestic rooms, it would be: In virtually any given room and any pair of main speakers,two subs perform twice as well as one sub and four subs perform twice as well as two subs.  
     As millercarbon stated, these conclusions are not based on opinion or belief.  They are based on standardized scientific experimental research, under controlled conditions, with results measured and recorded by accurate and reliable equipment.  
     You've stated that you believe one sub is best, that many believe 2 or 4 are best but not in every room or every system.  My point is that this defies the scientific research results that specifically were found to apply in virtually any given room and with virtually any pair of main speakers.
     In my personal experience with my own systems and assisting to construct other systems, I've invariably found that 2 subs actually do perform twice as well as 1 sub and 3-4 subs perform at least twice as well as 2 subs.  Why you would prefer 1 sub over 2 or more subs in your room and system is hard to understand.  It makes no sense to me not only because it defies the scientific findings and my personal experiences, it defies the laws of physics.  
     Well, it's really your big loss since you're the one who's definitely missing out on significantly improved bass performance with your system and in your room.  Of course, it's your choice if you're satisfied with the bass quality attainable by only using a single sub in your room.  If you do get the urge to improve your system's bass performance, however, I'm willing to assist if you'd like.

Tim