I have done a lot of research on subs an found that I like value and performance combination of Elemental Designs subs. I have an A3-300 on order now. Plus Elemental Designs has free shipping which saves you $50-100 depending on what you buy and shipping distance. There also no tax outside of Iowa - granted u should claim on your state income tax return (if applicable).
With that said, the SVS PB12-NSD DSP that will be shipping in early 2011 seems like a solid buy. SVS, HSU, Outlaw, JL Audio, Elemental Designs and Velodyne will be recommended the most.
My suggestion is buy the biggest you can afford. I am on my third sub in four years trying to get something cheaper and smaller - (HTIB sub, DefTech ProSub 60, Klipsch Sub 10). The A3-300 should suit me fine for some time.
I suggest posting in the sub forum section for more input