Subwoofer Choice

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.

+1 @lak , go for the S series. I haven't listened to the T Series, but have listened to the S Series and currently have a pair of Carbon Special.  A lot of T series are for sale on Audiogon and USaudiomart, wonder why they don't like the T series anymore.