Best Big Sub? or duals?

Value goes to the HSU. I would do 2 15's in that room, upfront near mains, possibly in center on front wall behind/near the center, run in mono (.1 channel). Better would be two Paradigm Servo 15's (musically even more accurate).
Other dual sub options available, obviously, (yes, I like dual subs for you, EQ'd, up front for best integration....FORGET THE SUBS AROUND THE ROOM TO SMOOTH RESPONSE WITH TODAY'S TECHNOLOGY - besides, it looks unsightly and cluttered around the room) but would likely run more for the quality. Earthquake dual 15's will run more, and not out-do the Servo 15's.
Did I leave any out?

Hey Coach, my room is big. 18'w x 40'd x 10h and one on each side of the speaker is ideal. Adds great bloom, depth and stage. Nice controls which makes them very versatile and integrate nicely with time. They don't disappoint that's for sure.

Jp1208, my room is comparable size except I have 8 foot ceilings. The adjustments, and automatic calibration is well worth it when it comes to sound quality are worth whatever it might cost above some of the other subs people have mentioned. I thought is sounded good, then I ran the auto calibration and it was like night and day! I used to have a Paradigm PDR12 which I thought sounded good with my fronts, but the Fathom blends even better with them.
Some day I hope to have a space that can handle 2 Fathoms. I think the structural integrity of my home would be compromised if I had 2 in it!

After much research and listening to them it was a toss up between the dd18's and the Fathom f113. I chose 2 fathom 113. It was just a smaller faster and a more powerful sub, the DD18 won in the software and set up I like tweekeing and the eq is awesone on it however the Fathom can be tuned almost as well just doesn't hwave the feedback the dd18 has and at the end oc the day the fathom was just a more impressive sub.

Feedback? From a subwoofer, what's that?