I wouldn’t run them together, but a pair of subs will go a long way towards a nice smooth response. Not sure how big the room is, although you seemed to question having enough sub, so I’d guess large. But I would never buy a sealed sub. Especially for HT.
Also, the 3000 series is a much better value than the 4000’s. There are also other sub manufacturers that are better value as well. Rythmik, PSA, monolith, and HSU, not to mention DIY which can really offer tremendous value and performance.