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?