My living room is an open floor plan like yours with a 2 story cathedral ceiling. I have an older SVS 2039pc+ (12” driver with 500w amp) that produces plenty of earth shattering low bass. I use a Dspeaker Antimode 8033c to tame the peaks and take away room nodes. The improvement is very noticeable with cleaner multi-note bass. Instead of multiple subs get one very good sub and the 8033c.
http://www.dspeaker.com/en/products/anti-mode-8033.shtml
http://www.dspeaker.com/en/products/anti-mode-8033.shtml