JTRs are easily the highest performers on that list, but they are also the most expensive. If you can afford them, a couple Captivator S1s would be pretty killer in a room like that. Or better yet, Captiavator 2400s. Captivators S1s will save you a lot of floor space.
I would rule out the SVS PB16-Ultra. Killer sub, but it has a very large footprint. A good SVS sub that will have a lot of deep bass output for a small footprint would be the PC-4000. For your situation, that would be my choice among SVS subs.
Rythmik FV15HP is a fine sub, although it is the only one I haven't heard from your list. It does have a pretty significant footprint though.
The Monolith 15" THX Ultra is a big sub that will eat up a lot of space. Like you say though, it does have near textbook-perfect performance. The Monolith 12" THX Ultra is more reasonably sized. It's still large for a 12" sub; It's not that much smaller than a Hsu VT15h mk2- but the Hsu is a higher performer. I would go with a Hsu 15" over the Monolith 12".
Two of any of these subs will get you massive bass in a room that small. Just massive. Something else to think about is getting three or four smaller subs to help even out the room response. Maybe take a look at getting some Outlaw Ultra-X12s or Hsu VTF-2 mk5s or SVS PC-2000s. That will still get you massive bass but three or four of them will go a lot further on smoothing out the room response than two behemoths.
Some more advice for a room like that:
It's a narrow room, and you don't want to put your speakers really close to the sidewall. However, if you are angling for a large screen, that might be unavoidable. If you have to place the speakers very close to the sidewall, I would recommend placing some broadband acoustic absorbers along the wall in front of the speaker. The thicker, the better. 2" thick absorbers would be OK, but 4" thick absorbers would be better.
For seating, the existing arrangement, as you have surmised, is suboptimal. Having seating so far to the left or right basically means there isn't a good seat in the house. You will want a set up with a centered seat, a "sweet spot." If you don't want to look too selfish for your family, use a loveseat type sofa or something like that. One thing people do a lot that wrecks the sound is they get these big home theater seats. There are a bunch of problems with these as they relate to sound. First, the high back blocks your ears from the surround sound speakers! A lot of dudes will buy big, expensive, electrically adjustable seats that end up defeating the point of having a surround sound system! Don't get anything that has a huge headrest.
Second, many of those seats have leather upholstery. But many types of leather will reflect sound much better than they will absorb it. So it's like listening to a sound system with an acoustically reflective surface right behind you. That does no favors for sound quality. Again, another argument against high-backed seating.
The room is quite narrow so I would think about just using in-walls for the side surrounds or else the speakers can really get in the way.
So you have Polk LSiM 707s. Those speakers have a crazy low crossover to the bass drivers at 100Hz. I would try to take that crossover element out of the equation as much as possible, by using a high crossover frequency for the subs. Experiment with a 100hz or even 120Hz crossover to the subs.
Before you had mentioned going 9.x.4 instead of 7.x.4. There are some advantages of the front wide speakers in a 9 channels bed system, however, I think your room is just too small for that. I would go with 7.x.4. Try to use in-ceiling speakers as the heights if possible, but I suspect you already know that.
If you have the money, you really should think about getting the Monoprice HTP-1 for a processor. Let's say you get a 7.4.4 system; the HTP-1 can independently calibrate up to 5 different subwoofers, using Dirac's bass control system Dirac's bass control for multiple subs is a $500 upcharge though, but if you want to calibrate your system to perfection, it's worth it. The HTP-1 supports that feature.
Another argument for getting a processor like the HTP-1 is that your speakers really benefit from an outboard amp. A Polk LSiM set is going to put a lot of stress on the amp in an AVR. You will want an amp that is comfortable with low-impedance loads. Outlaw Audio and Monoprice Monolith have some great affordable amps. I am sure Emotiva does as well.
