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!