Subwoofer Choice

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.

I own Sopra No2s

Congrats on the purchase of the No3s.  I have a fully treated room w/ bass traps and a JL F112v2.  I find that the Sopra's alone do a great job with a good recording.  If the recording is thin I'll activate the sub to correct the recording.  I don't pay attention to stats, they mean nothing to me.  From real experience, properly positioned and adequately driven, the Sopra's will make a ton of bass.  They are front ported and have a good deal of speed IME.  Be sure to match the speed with the sub.  Speed isn't about size, it's about quality of the driver and power/electronics.  I have two (2) SVS SB4000s in my HT room and IMO the JL is considerably faster.  I the JL makes more power and has a large toroidal transformer that the lower priced SVS does not.

Probably more than other speakers, the Sopra's are sensitive to the grip of the amp.  Pairing your No3s with an amp with a very high damping factor will give you crap ton of clean, fast bass.  

I would get the new speakers in place, run them in, and then decide what your needs really are.  Set up properly with a good source the No3s will be darn near full range.  Again, I always disregard specs and trust my ears.  Good luck and cheers.