With a room this size and being open, finding a small 8" sub that delivers what you want will be hard to find. You'll probably have to go with a ported design and a bigger box, that's quite a bit of air to move. I have heard the EP500, it's very good and is not that big.

Bruno
M80s/VP180/QS8s/EP600/AVR-890
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"The problem is choice..."