Sub Confusion

I've decided to add a REL sub to my in-den 2-channel system. My room is 12'-0 x 15'-6 x 10'-0h with PMC Fact 12 speakers 3'-0 out from the short wall. I have GIF corner base traps (43"h) in the front corners. My integrated is a DarTZeel LHC-208, and my DAC is an Aqua Formula xhd rev 2. When I go to REL's website to match a sub to my speakers, they recommend a 212SX (at $4,999). For my space, these seems to be overkill. Why not a single S510 (at $2,749) or two T7X (at $1,099 each)? Or? My goal is not more base- I'm looking to free my PMCs from having to expend energy to produce sound below 80 Hz and increase depth. Looking for input from our members. Thanks