I'd get 1 good one then save up for a 2nd down the road. SVS, Hsu, Rythmik, RSL... all good choices, and I know there are a few more I'm not thinking of right now, but anyhoo. Hsu is about your best bang for buck, imo. They're an id company that specialize in subwoofers and their main goal is performance, where some others also put a lot into finish. Which is nice, but also bumps up the cost.
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The size of your room would be a huge determining factor in picking one out. How big is it?