Several good brands are available on-line and offer a free in-home trial period. I'd agree with Lloyd that your space is large and that a subwoofer (or two) would likely improve the tonal balance of your system, but a free trial allows you to satisfy yourself.
One very appealing offer comes from SVS (svsound.com), which shows a pair of sealed 12" subs for $950 (last I looked). The SVS subs have an excellent rep, and a pair of subs is likely to get you smoother in-room response than a single sub. If you do want to stick to one subwoofer, you could check out Rythmik (I use two and am completely satisfied) or HSU (another brand with a good rep).
One very appealing offer comes from SVS (svsound.com), which shows a pair of sealed 12" subs for $950 (last I looked). The SVS subs have an excellent rep, and a pair of subs is likely to get you smoother in-room response than a single sub. If you do want to stick to one subwoofer, you could check out Rythmik (I use two and am completely satisfied) or HSU (another brand with a good rep).