Whoa, that's a big room. Mine has a similar footprint, but with a cathedral ceiling and it opens to a two-story foyer. After living in this space with only one SVS-PB12ISD for a few years, my opinion is that big spaces can benefit very significantly from multiple subs. If I want to actually acquire a second sub, it's going to need to fit in better aesthetically than another black box, so I'm seriously considering a DIY thing.
That being said, I think a new TV is WAY more important than a second sub.
We are a whole community of "that guy" - StPatGuy