Subwoofer advice


So looking to add a new component (or 2) to my system (Snell Type D, McCormack .5 or Jolida 502b, Schiit Saga pre, Rega Planet, Dual 1219 TT) and contemplating a sub.  Basement room is 14'w x 30'l x 7.5'h and treated with first reflection points and base traps on front wall.  Interested in the REL T series, either the T7i or T9i.  The Snells have 8" woofers so not sure the T7i at 8" will make a significant difference.  The room might be too big as well.  So it might have to be the T9i at 10".  Looking to supplement clean low end not anything bloated or overbearing.  

Any thoughts greatly appreciated!
pkatsuleas
Whatever subwoofer you choose and however subwoofers you decide on, if you don’t want to turn you room into a giant shaker table experiment, isolate the subwoofer(s) from the front end electronics by placing them on springs. Problem solved! 🤗
I've had good results with Auralex SubDudes under the subs for isolation.

My experience with high-pass filters may be Ayre-specific, but I tried a Bryston 10B active filter and a Marchand fixed passive filter (4th order 80 Hz), both claimed to be balanced.  The Bryston really changed the sound, the Marchand removed a bit of the sense of air.  Both are gone.  I let the KEF Ref 1s run full range and supplement them below 40 Hz with pairs of Velodyne HGS-15s and HGS-10s and SMS-1 bass managers that provide acoustic room correction.
3. Best Performance system- Audio Kinesis 4-sub Swarm for $3,000. This option will clearly outperform the other options since it will provide deep, fast, smooth, accurate, highly detailed and dynamic bass that will be perceived as the most effortless, the most natural and the best integrated with your main speakers. It will also be the most convenient option to configure since all settings are made once on the amp/control unit for the 4 subs collectively rather than separately on each individual sub. 

Good luck,
  Tim
   I've respected your detailed and informative suggestions helping people new to subwoofers by sharing your basic knowledge and your enthusiasm with this system. I'm saddened to read your declarations of, "Best, most, and outperform the other options."