With that size room it is not so much you need to spend alot for a sub, but you need to get the most sub for your money, HSU Research makes fantastic subs that put many to shame at a fraction of the cost. For example I had a Definitive Tech 15in Powered super cube (6 or so years ago it was their statement piece and a very good performer), I ran across an HSU Research
12V powered sub and it was way more punch, control, depth and doesnt call attention to itself and it was less than half the cost. Dr. HSU is an extremely helpful but somewhat hard to understand owner who actually answers calls.