I have an EP600 in a 4000 cubic foot room in the basement. It fills the entire room with deep, heart-pounding bass...no second sub needed! During LFEs, the entire house shakes from top to bottom. There is no escaping the power of this one-eyed clown anywhere in my house. And I have the volume on the sub set half-way and the LFE gain on the amp has headroom for another 22 dB! My sister was over last night to watch U-571 and she was severely freaked out and distressed during the whole movie (as she put it, the audio felt like she was in the middle of the action) but particularly during depth charge scenes.
So based on my experience, one EP500 for your space should be more than adequate.
