With main speakers that reach down as far as the Legacy Focus, the primary reason to use subwoofers is to smooth out the bass, as opposed to giving you more of it. Unless you get lucky, you might find that positioning your Foci for best imaging doesn't result in the best bass response.
Your room really isn't very big, so a massive sub like the DD18+ is probably overkill - IMO what you should go for are two smaller subs, like the DD12s, and place them in the room for smoothest bass response. Of course, two big subs like DD15s or DD18s are good too, but two Foci and two DD18+ may look rather overpowering in a room of your size. Not that sound will be affected, big subs don't sound overpowering just because they're big, you simply run them at the appropriate level.
My recommendation is not to order any subwoofers until you have the Foci in your room. I'd invest in some measurement software like the OmniMic system from Parts Express, and measure the smoothness of the bottom four octaves. The Focus, with its crossover at 120Hz to the 12" drivers, already has a built-in subwoofer, so to speak. I think there's a good chance they may suffice. If they don't, order a single subwoofer - that you can afford two of - and hook it up. You'll probably want a 40-60Hz low-pass filter point on the sub. If bass octaves are now smooth (+/- 5db is about what you're looking for) you're done. If you still don't get smooth bass response, order up another sub. Now you'll have four bass sources, and by spacing the subs around the soundstage space you'll have a very high probability of getting smooth bass response.
I use main speakers capable of very deep bass too (Salon 2s), but I had a lot of problems with smooth bass response. My room has a much larger effective volume than yours, so I chose the DD18+, and by chance I got lucky, and one DD18+ gives me very smooth bass response.
I like the Velodyne DDPlus series because it has so much adjustment flexibility that other subs, like the JLs, lack. For example, I use a 40Hz low-pass crossover point, but Velodyne gives you the choice of a 6db/octave roll-off, which means that at 80Hz the Velodyne is 6db down from the 40Hz level. That gentle slope meant better blending with the Revels, according to the OmniMIc, than the 12db/oct or 24db/oct settings, which are the only two the JLs offer. Blending subs and mains can be a trail and error situation, so the more options the better chance there is of success. Of course, starting with full-range speakers like the Foci makes it easier too.
Of course, this might be a moot point, if you're using electronics with bass management, but if you're not, if you're using audiophile 2-channel stuff, then the wide array of filter and slope options (not to mention the EQ capabilities previously noted) may make the Velodynes a better choice.
Just don't assume one sub is enough to get smooth bass response. And you're shooting in the dark if you don't measure.
