Subwoofer Choice

Why not Svs , sealed boxes are more accurate then ported or vented which Rel 

isit has a passive radiator on the bottom , I have heard  most of their models up to $3k

the most accurate which my brother has. Is the JL audio. Fathom 12 

whichhas full room correction built in. $5k retail which btw is Sealed 

the SVS ihad the SB -sealed box 3000 which was very respectable and great app for adjustment at $1k. I moved to to the SB - 4000. Which is a Big step up in output 

and tunefull accurate bass Rel cannot match especially in the $1700 range 

and  tons of bass slam not that big but very dense 102 lbs ,the driver alone os around 50 lbs , SVS gives free delivery , and a 45 day audition , plus a 5 year transferrable warranty the others Donot.  You have nothing to loose and a very good app to adjust from your chair , not the back of the sub .

IMO with your system you need to consider what will play better with your components. One thing that stands out to me when looking at specs is the amp design of the subs. Rel is class A/B and SVS is class D. I tried both and went will the Rel as I just could not get the D amp to disappear in my system. When listening I could always locate the sub no matter where I placed it. Then I tried the Rel and it just became an extension of my Vandersteen’s. Subs are self powered so don’t over look the amps difference between brands.

Audition the RELs and put your hand on the enclosure. You will feel it shaking. That is distortion. Audition the Martin Logan and KEF balanced force subs. When you put your hand on the enclosure you will feel almost no shaking. These are significantly superior designs. The KEF even has a full two way crossover. Magico uses a similar design but they are very expensive. 

You have a fairly large room. My room is also 16 feet wide and I use 4 12" subs. The absolute minimum is two 10" units. I would not go any lower than two 12" subs or equivalent. You will also need a complete 2 way crossover. 1/2 the benefit of using subwoofers is removing the bass from the main speakers. This allows you to use a higher crossover point and significantly lower distortion in your main speakers along with adding up to 6 dB of headroom. Most commercial subs only have a low pass filter. You can high pass the main speakers by soldering a capacitor of the right size in series with the input. The capacitor value depends on the input impedance of the amp and the 3 dB down point (crossover point) you want to use. C = 1/RF Digikey will sell you any capacitor and your amp's manufacturer can tell you the input impedance (R) of your amp. 

I have two REL T9is.  I think they are particularly musical, well built and a great value. I’ve tried other stuff but kept coming back to REL.  I do wish REL had an APP but I get their reasoning for not going that route. 

Last year I purchased the SVS 1000 pro  and a Polk Audio dsw micro 3000 in a 14 x25 foot room and for me it wasn’t enough bass.this year I purchased a monolith with dual 10”woofers and it fills the whole room with no problem.