Single vs. Dual Subs

I’m not a real sub person for stereo, only for my AV system, which I have two. I started with one and noticed a vast improvement with two in terms of balance. 

xcool - Our ears aren't good at locating low frequencies so subwoofers don't contribute to the stereo image. In fact you will probably find that the best locations are not near your speakers or symmetrical.
What you're looking for is for each sub to excite different room modes so that they balance one another out to some degree. If you're not familiar it might be worth looking up 'room modes' and 'standing waves'. 

Excellent answer. My system is a great example. There are 5 subs asymmetrically placed around the room with each one a slightly different distance from a corner than the others. With music playing, even with plenty of bass, you can walk right up to any of them and think it is disconnected. In fact I wasted a lot of time in the beginning doing exactly that! 

HOWEVER in spite of this when listening to music it is incredible how the bass always has some sort of definite location or character. What makes it so incredible is this happens even with bass notes that seem to have no sharp transient or leading edge. Like a big drum whack, it makes sense your ears are cued to a location by the initial whack on the skin of the drum. Or pluck string bass, same thing, the initial string plucking off the fingertip. But all kinds of bass does this too. It SEEMS to be localized BUT it is NOT and the proof is I can run the amps mono and STILL hear the exact same localized bass!

None of this makes any sense other than our hearing does not localize very low frequency bass but does localize higher bass AND THEN combines the one with the other to form a clear image of where the whole thing is coming from.

In other words, a great example of psycho-acoustics.

Everyone needs to go to another live concert and ask this question (assuming the sound system is a good one) Why does this sound better to me. Why does this feel better to me. Jazz in a smaller venue is perfect. The idea is to recreate that sound and that feel in your room. Having a room longer than 30 feet is a big help but not mandatory. A lot of systems have the capability of doing this. What is usually missing is the bass. It requires a lot more than people realize, a lot more speaker and a lot more power. I just get there using 7000 watts and four 12" subwoofers. It requires a 2 way crossover with steep slopes and the ability to delay speakers to get them aligned correctly. Just adding bass is not the point, it is getting to realistic bass. No speaker that I have ever heard had realistic bass down to 20 Hz, not a Wilson, Magico, Magniplanar, or Apogee. It takes a specialized driver in a very heavy stiff enclosure and a lot of power. You could use one really big one but IMHO it is easier to live with multiple smaller ones than one huge one. In my case it is absolutely necessary. IMHO if you are going to add subwoofers wait until you can do it right. It can be very frustrating if you don't and waste a lot of your time.
@erik_squires , forget about simulations. Measure actual setups and see what they do in reality. A simulation can not possible cover all the variables involve like what frequency the walls resonate at. Are the subs well matched or are they 90 degrees out of phase with the main speakers. Why do some people think their subwoofers sound better 180 degrees out of phase? One sub may do fine in an 8 X 10 foot room or as fine as you can get in a room that small which is not very. Do not make up your mind on someone's idea of what a simulation should be like. 

You can't recreate the sound field of a live event in your room with a stereo system. It can't even be recorded. The best you get is a good facsimile. 

My system is a great example. There are 5 subs asymmetrically placed around the room with each one a slightly different distance from a corner than the others. With music playing, even with plenty of bass, you can walk right up to any of them and think it is disconnected. In fact I wasted a lot of time in the beginning doing exactly that!

Just for the record, I do this regularly with a single sub, traps and careful use of an EQ.