You never stated a budget and I hope it's a good one.
Based on those pictures looks like a lot more space than just a 20 x 24. Subwoofers work on total volume.
The guy at B&H was right about the SVS recommendation. But his thoughts of PB-1000's is far off. Dual PB-3000 would be a better recommendation if not Dual PB-4000's.
SVS was a small company in Ohio making subs for home theater for years operating as a Internet Direct company. The company was sold and as expanded outside Internet Direct, but still produce outstanding subwoofers. Their build quality, their quality of components drivers and tweeters. Their fit and finish, and their customer service is second to no one.
Powered SVS home theater subwoofers unleash the ultimate bass experience for music and movies.
www.svsound.com