It's a really tricky question to answer. Honestly, I don't know enough about acoustics to give you a perfect answer. But here's a thought, instead of adding more bass input with a second sub, why don't you add some bass traps to help tame some of the bass in the room? For the same cost of a new sub, you can either purchase some bass traps or make your own. Here's a great website for ideas:

www.realtraps.com

Hope that helps...