Best type of Sub to consider?

The guys at Rythmik quickly got back to me, so they checked out my system and room layout relating to integrating the OB/Dipole subs that have been recommended above. Unfortunately, they did not recommend the OB design with my dynamic speakers as they work best with planar speakers.
What is interesting, it was suggested adding a small sub upfront with a larger sub in the back. I'm now working out those details, because with two subs, I was looking for two channels. I'll wait to hear back and let you all know.
Kenny

Interesting. Just for additional background, I bought a 12 inch sealed unit from Brian at Rythmik recently, opted for the paper, rather than metal cone, and for cosmetics, had it finished in that piano ebony gloss (which may not look right with your aesthetic). I got the version of the plate amp with XLRs that allow me to daisy chain (which I'm not doing right now). The other woofer I'm using is a very old Velodyne 15" to which I added a small DSP unit, and cross it over really low. I am running the pair in stereo, adjusted to match the output of the 4 x 10" woofer array in my Avantgarde Duos. I am not rolling off the main speaker system through any external X-over, but running the subwoofers parallel to the main system from additional outputs on the back of my line stage. Although, as described, it is sounds a bit kludged together (and it is), it has worked very effectively. So, the idea of using two different sized subs isn't completely crazy. (I have another old sub, a Velodyne 18", which I didn't bring into the room- it's about the size and weight of a fully loaded coffin and it just seemed to be too much, and unnecessary). I am using the set up described as a stop-gap until I move and rebuild the system, where I'll revisit the whole speaker array. (Bigger horns, please!)  The bigger woofer sits further back into the room, behind the "line" of the main speakers and off in a corner; the Rhythmik is closer to my listening position on the other side of the room at a "jog" in the room so it has some "corner loading" but it is more mid-wall, if that makes any sense. Using some pretty simple tools, including a db meter, some of the basic measurement tools that come with that Studio Six app for iPad as well as my ears, I was able to adjust the volume, crossover and phase in a way that pretty harmoniously adds weight, depth and gives me more of a front to back effect without doing violence to the glorious midrange of the X-overless mid horn and SET amps.
For what it's worth, my dealings with Rhythmik were very pleasant, Brian was responsive to my questions via email and phone, and the fit and finish, as well as the performance of the 12" woofer is pretty impressive for the $. So I guess you could call that an endorsement.

No surprise that Brian and Enrico at Rythmik steered you towards one of their "normal" subs (sealed or ported?), as the OB is more of Danny Richie's (GR Research) baby. No matter, a Rythmik is a Rythmik! I predict you are gonna be pleasantly surprised by how clean, clear, and "quick" their subs are. No boom, no bloat, all muscle, no fat. The Rythmik controls make integration with your mains invisible---no seam to be heard. Brian's servo makes Rythmik's the fastest subs on the market, bar none. Sterling Sound in NYC has multiple F15's in their monitoring systems, the ones George Marino listens through when doing his mastering for Mobile Fidelity and others.

George has passed, but there is probably a good mastering suite in heaven. 

Um, make that "listened".