Two subs in a small room....

I have read that two subs smooth out the bottom end and give a much better sound. But, what if you have a room that is just not big enough for two subs. Is it worth the effort to get two "small" subs or is there a special placement for just one sub besides the usually place, behind one tower. What about placing the sub right in the middle?

By the way, my system is in a 10 X 10 room and it is in a diagonal configuration if you can picture that.
I have two Martin Logan Dynamo 10" subs in an 11x20 room. I have the subs placed on the short (11') wall. From my experience, if you can make one sub work, you can probably make two work, but one will probably serve your needs in that size room. I would recommend a Velodyne SMS-1 sub equalizer to aid in integration. Also, I would recommend setting the low pass filter just below the lowest rating of your main speakers, so that the sub basically just fills in the low end. Also, show restraint in the volume control.
To incorporate Duke's and Bob's comments into my previous post.

1) An optimallly placed single sub together with a bass restricted main speaker will usually produce smoother bass than a full range speaker that has been placed for best overall performance. (The best spot for bass generation is usually different than the best spot for generating mids/highs.)

2) Optimally placed multiple subs usually allow smoother response than an optimally placed single sub.

3) Room EQ can achieve smooth bass response with freestanding speakers or with subs. If you don't want to EQ your main speaker's signal (that would be audiophile heresy!), think about room EQ for one or more subs.

Personally, I use 2 asymmetrically placed subs and the Velodyne SMS that Bob mentioned for bass EQ. My main signal path is interrupted only by an NHT active x-over (that I can't identify in blind A/B tests) which feeds my main amps and the SMS, which is turn EQs and feeds the subs. I get outstanding results - both measured and perceived.

I'd also add that the SMS has a built-in room analyzer that makes optimal placement of the subwoofers a hell of a lot easier. The SMS includes an active x-over, can control up to 3 subs and costs app $450. IMHO, it represents great value. I'm not crazy about the unit's built-in, basic (digital) high pass filter, so I added the NHT x-2 active x-over @ app $300. So the total budget for my sub control set-up is $750 plus the cost of sub(s).

You can either buy cheaper subs and apply the savings to fund the SMS and/or X-2 or you can always start with sub(s) and add these control unit(s) later, as budget permits.

Either way, I believe that you'd be going down a good road.

Good Luck

Ok, so, if in a small room, a single subwoofer will suffice, what kind of small sub would be best? Ported? Sealed? Down-firing? And is there any sub that anyone could suggest?
I was going to write a long reply, but see that Martykl has already written more or less what I would have said. So I'll tick an agreement with him, and summarize in different words:

Multiple subs placed using a moderate amount \of trial and error will nearly always provide a smoother, less erratic bass than would a single one. The biggest problem with a small room or a large room isn't its size but just that it is a box with incredibly irregular low frequency response from all the multiple standing waves. Multiple subs let you choose placements that compensate some of the unavoidable holes and peaks that any of the subs singly will give you.
Ok, so, if in a small room, a single subwoofer will suffice, what kind of small sub would be best? Ported? Sealed? Down-firing? And is there any sub that anyone could suggest?

In a small room, a sealed sub will likely be easier to integtate. SVS makes a nice 12" sealed sub that has lots of flexibility at a decent price.