DUAL SUBS IN THE $10,000-$15,000 RANGE

 Plus the 'Paradigm Signature Sub 2'

Deep sea sound

Those are all great choices. Another is the Rythmik F25 at $3k (plus shipping) for a pair, $6 for a quad, $9k for a sextet, $12k for an octet, and $15k for ten (a dectet? ;-). Rythmik's 10% discount for a purchase of multiples will pay for the shipping. The F25 features two aluminum-coned 15" woofers controlled by designer Brian Ding's patented Direct Servo Feedback circuitry, each driver powered by it's own 400w amp. That is less power than that found in some other subs, but the Rythmik driver is unusually sensitive/efficient, not needing as much power to produce a given SPL as the woofers used in those subs. The TC Sounds LMS Ultra 5400, for instance, is extremely insensitive, needing far more power than other woofers to reach full output. The Rythmik F25 is a very sophisticated, high-performance subwoofer, and an absolute bargain.

If you are dropping that much, you should consider consulting a sound engineer.  You may get more bang for your buck by running different size subs to cover the frequency range better (12, 15 & 18 inch)?  If this is mainly theater, putting transducers/bass shakers under every seat might get the effect you want.
Personally, I love the story from about 10 years ago where a guy in Italy (I think) built 24 inch subs into the foundation of his house.  Imagine horn loaded subs under your floor built of poured concrete...

Another option to consider at your price level. Something different.   
Eminent technology rotary TRW-17 subwoofer

http://www.rotarywoofer.com/

http://www.soundandvision.com/content/eminent-technology-trw-17-rotary-subwoofer#Lpu6qStoHOLVAQpm.97

regarding the DIY with multiple TCsounds LMS ULTRA 5400 18"'s

Do you have a carpenter/speaker builder who will build the cabinets?
I am looking at a couple of subs as well for one of my rooms and am currently exploring the DIY route with a local custom speaker builder. I could bring him the parts; he builds the cabinet and I or he does the assembly.   
One of the cons I can see with this type of approach, if things change down the road  - resale.

I too am looking for a pair of subs to add fill-in for my Horning Aristotles, since I have open room corners. I was pointed towards M.J. Acoustics.

Why don't you go to the Audiokinesis website and read about the Swarm subwoofer system. You may get a better result and save a lot of money. 

Thanks for the reply/ideas guys. Deep sea sound look to HT oriented plus I want to get away from sealed wood enclosure which the majority of the subs are. The Gotham's are not & you can hear/feel the difference. The Rythmik F25's look like great subs & are priced very reasonable. They also address box resonance issue with their patent direct servo technology. I'm sure the bass would be very tight & controlled. Great website too. Rotary woofer only goes up to 20HZ so I would still need subs from 20HZ-60HZ. Regarding the TCsound 5400's my friend(who is a acoustical engineer) wants to help me build dipolar line-source subs to match my Apogee's dipolar design. Three 5400's per side using Ikea butcher-block tabletops(which you can by in bulk) as a baffle. I've good & bad about dipolar subs regarding low bass & room cancellations. Not sure if I want to go down that road. After all is said & done, I'm counting on what I heard/felt(my pants were flapping & hair blowing!), all the awesome reviews(ultraaudio.com), too many patents to mention, the ARO room correction build in, the fiberglass enclosure, ability to master-slave(big plus since my equipment is on the side of the room), taking a month to build in their state-of-the-art facility, plus their reputation that the Gotham is the 'MOTHER OF ALL SUBWOOFERS'!

Just so you'll know, the Rythmik Servo Feedback plate amp is available in two different XLR versions, one of which (XLR2) allows master-slave hook-up, operation, and adjustment of multiple subs. The feedback system also deals with the resonance of the MDF enclosure, as well the thermal artifacts endemic to ALL drivers. Those effects are the result of the electrical characteristics of all woofers changing as their voice-coils heat up and cool down in normal operation. That's just the tip of the Rythmik iceberg. As I said, very sophisticated! The Rythmik website goes into great detail explain Brian Dings design.

I'm not current on what's "hot" and surprised the REL Gibraltar
doesnt appear to make it in at least the top 5 subs?

I have a simple system with  an older B3. Is the current lineup of REL not competitive? The wireless w/remote seems like the ticket, do the others offer this?

ptheo, thank you for sharing the magico s-sub info.  I wonder how much bass one needs in a typical HT or 2-ch setup? How big is your listening room? 

 I just installed a f212v2 last month in my HT room (15"Dx30"W).  Just one of the 212's is freaking amazing, I can only imagine two of 212's or a pair of Gotham's in my setup.  

lallitk- My room is 27'D-17'W. My complaint is not the output of my F212's(mostly 1/3 up & never past half way), but the way the drivers were working so hard once I cranked it up for movies(Mission Impossible- Rogue Nation beginning airplane screen) which causes distortion to rise which causes the MDF box resonate more which colors the sound/output. In my demo with the Gotham's, I got none of that. Just hard hitting clean/low distortion bass with no box resonating/muddying the sound. Sold!

Hmm...I am going to cue up the opening scene once I get home tonight and try to hear what you experienced. 

Also, the opening scene of Edge of Tomorrow has some serious bass, I think it gets down to 10HZ.  

Thanks!!