If the main issue is output and budget for maximum performance, one 500 will be more than enough in a 2300cf room. If other concerns include more even FR and fewer flatter room nodes, go duals. I found 2 600s or 2 500s in my 3500cf HT provided a significant step up sonically while keeping total output constant. There were marginal improvements with adding more subs but not enough worth recommending. In your 4000cf room, Mojo there is no doubt you would benefit from adding a 500 or another 600. The bass would be that much more palpable and articulate, though ideal installation would require more effort.
