Subwoofers

I'm looking at subwoofers, my speakers are Sonus Faber Olympica III. I have looked at a lot of subs to the point of overloaded. I'm leaning toward REL S812 X 2 due to room size. My living room, kitchen and dining room are open with a 24 foot vaulted ceiling, area is 35 X  25. 

JL Audio, Rythmik, PSA, HSU, to name a few have been checked out. 

HELP!