Thinking of adding a subwoofer -- my learnings

I am thinking of adding a sub to provide sub-30Hz frequency response mostly for music but also for the odd DVD. With so many options from which to choose, I would like to (i) share my informal research findings and (ii) ask people for any additional insights or learnings not covered below.

Three principle issues that need addressing are as follows:
(1) downward firing vs forward firing sub
(2) single sub vs pair of subs
(3) required sub output level for a given room size
Of course a 4th issue is placment but this is after the sub is bought so I'll leave that for now and focus on what info is needed to make an informed buying decision.

(1) downward firing vs forward firing sub
* downward firing loads the room with bass just as effectively as fwd firing
* downward firing subs help use the flooring to filter out higher frequencies better than a fwd firing sub would. Even if your sub's low-pass crossover filter is set at 40Hz, the filter doesn't act as a brick wall and prevents a 41Hz or higher frequency from playing; Instead the 1st or 2nd or 3rd or 4th order cross-over filter still passes higher frequencies through the sub but at diminishing rates - 1st/2nd/3rd/4th order crossovers pass higher frequencies at -6/-12/-18/-24 dB respectively. So what you get with a downward firing sub is add'l low pass filter effectiveness. Think of tipping your main speakers forward 90 degrees so that it's now downward facing - you'll still hear the bass but the treble and midrange will be more muffled due to the downward firing direction.
* with small children, a downward firing sub doesn't provide a grill cloth to tug at or open port to put toys down - more family friendly

(2) single sub vs pair of subs
* for home theatre a single sub will suffice but for music the audiophile thinking is that a pair is best because soundstage ambience cues like reverb or echo from say churchs, cathedrals, concert halls etc will be more pronounced with two subs than one sub with the net result being a larger soundstage. Most recordings use stereo mics so there should be low level ambient cues in each channel hence the need for a pair of subs to render such cues more audible.
* a pair of subs will play louder than a single sub, all things considered. A pair of ACI Force subs will play 6dB louder than a single Force sub. A single ACI Titan will play 3dB louder than a single ACI Force but two Force subs will play 3dB louder than the single Titan sub.
* a pair of subs loads your listening room more completely and evenly than a single sub will, meaning the bass will be more even throughout the room with less boominess and null areas. A pair of subs could mean that you won't need any equalization that some manufacturers offer (e.g. Velodyne DD series). This equalization is really geared to work best for a given listening position so if you move away from said position the benefit of the eq is lost, hence the benefit of adding a second sub.

(3) required sub output level for a given room size
* this is a tricky one because it depends on the room size, how loud you listen to music/videos, the size of the woofer cone area, size of the woofer box, and whether the woofer is a sealed or ported design. Best to ask each manufacturer directly what they recommend.

What I'm leaning toward is a pair of downward firing subs from either ACI (Force or Titans) or MJ Acoustics (Ref 200). Any other suggestions or comments on firsthand experience that you'd like to pass along to help others with their buying decision?
Have you done any analysis on interference/cancellation with two subs?

For example, let's say a given recording from a concert hall is mixed to two channels that portray a sound stage of a certain width. The low frequency waves would be almost the same from each channel. If you have two subs placed too close together or too far apart, their outputs could cancel or interfere in a non-musical way.

If you are using only one sub, you might be driving it with two line level inputs. Many subs blindly deemphasize one channel in favor of the other so as to avoid any possible cancellation effect. The presumption is that the low frequency signals from each of the two channels are essentially the same except separated spatially by a fraction of a wavelength, and simply adding them even at the line level would result in unwanted cancellation. Personally, I don't particularly like this artificial signal processing.

I have two subs placed in close proximity to my two main speakers, each driven by the line level signals from their respective channels, relying on the built-in low pass filters to isolate the low frequency band. My goal is to simulate as much as possible having two full range speakers that go all the way down, but I don't know if this is the right way to place subs. Notably, my main speakers are placed to optimize soundstaging and dispersion for mids and highs, but I would be lucky if these locations were also the ideal places in my room for good low frequency playback. I hope I can learn something from the other posters.
Good post Jameswei but no I haven't come across research related to interference/cancellation with dual subs.
Single Rel Storm 3 works great in my system. Downward firing, with crossover set at 27 hertz. No longer made, but readily available used. I've been mostly UNABLE to localize it, even though it's in the corner. Deepest bass usually seems to emanate from the middle of the two speakers, even though that cannot be happening. A wonderful and sometimes hilarious thread regarding the merits of one versus two subwoofers has already been posted. Aside from the entertainment value of the arguments, what I took away from it is that stereo imaging becomes more of an issue above 60 hertz, and that it's less important below that frequency, because the deepest bass is nondirectional.

One versus two subwoofers

I don't know the answer, but can verify that in my system a single sub-bass unit suffices insofar as imaging, with the deepest bass seeming to come from exactly between the monitors, or sometimes JUST off center towards the side with the sub, but still between the speakers. I'm talking about a perceived shift of only one to two feet. I don't claim to fully understand it and am open to the possibility that my auditory perceptions are flawed.
I posted a thread on the 15th of last month (Dec.) that almost mirrors yours.
Subwoofer : Front Or Downfiring For Musicality ?
Considered the same sub, MJ Acoustics Ref 100 or 200.
I auditioned REL, James, Velodyne, M&K, all very nice subs.
Then I listened to the Definitive Tech. Super Cubes 1, 2 and 3, search was over.
Ended up getting a pair of Cube 2's, couldn't be happier.
It does not sound like "added" subs at all.
Clean, tight, musical bass, that play low and blend well.
They'll go loud (if needed) but I'm only attenuating my system at moderate listening levels.
I don't know for sure, but, I suspect a downfirnig sub would more likely have more potential to excite room structures such as a less than solid floor. Not to mention excite neigbors below.
Have you considered the Vandersteen 2WQ? This one invariably comes up when subs are discussed & typically gets glowing recommendations.
I'll share my thinking in case it helps you. I'm contemplating the same upgrade - going from 1 Kinergetics sub to using a matched pair of subs. I've read other posts here about this subject and Mdhoover is right, the thread he refers to is hilarious but does have some useful information.

It seems from the posts that the people who do use 2 subs swear by it and the people who don't seem to be of 3 opinions: 1. those that aren't sure of the benefits of stereo subs, 2. those that think it's unnecessary (1 should suffice in most rooms), and 3. those that think it's detrimental (cancellations, etc.). I'm currently in the "1" camp but am willing to experiment and keep an open mind. I know I want to get atleast 1 good hi-end musical sub such as REL, James or the Martin Logan Depth but am considering trying a set (ACI, Vandersteen 2QWs).

The 2QWs have 3 8"down firing drivers in each of their sealed cabinet subwoofers (I've heard sealed subs are better for music vs the ported designs). But the 2QWs use a line level crossover between the preamp and amp and requires you to connect the subwoofers via the speaker outputs of your main amp (you add separate speaker wire connections on top of your current front channel speaker connections and their system somehow knows how to direct the low frequencies, based on adjustable crossover settings, to the subwoofers' internal amps while still passing on the signals to your right and left speakers). This concerns me a bit but Vandersteen says it doesn't change the impedance your amp detects, although they do need to know the input impedance of your amp (to make sure their system will work with your amp?). And I'm not sure if the low frequencies directed to the subwoofers are also sent to the left and right speakers but I think they're not.

What I do know for sure is that, IMHO, a good quality subwoofer adds quite a bit to the listening experience by adding the lower octaves which increases the presence, tonal balance and realism of music. Hope this helps a bit.
Thanks everyone for your comments.
* A retailer told me that connecting the sub using high-level speaker connections from the amp wouldn't be a good thing given my 6watt/ch. SET amp so line-level connections from my preamp are a must - something I hadn't thought of. He said that the add'l load would be harmful to the transformers.
* It was also mentioned that the reason for getting two subs is to load the room more completely than a single sub could.
* He also felt that downward firing subs wouldn't play as loud given the small clearance to the floor that most downward firing subs face (I don't know if I agree with him).
* He also said that because my main speakers are very efficient (97dB) it means getting a sub that can keep pace with it.

Due to CES not too many retailers were around the past few days . . . Any Audiogoners see/hear any stellar subs at CES?