These are all great subs at great prices. I almost bought one, but at the advice of many I ended up going the DIY route. For about $1,300, I bought two Dayton Ultimax 18" subs, with sealed flatpacks, and a Behringer iNuke6000DSP.
I would have been happy with one or two of any of those subs, but these Daytons outperform every other element of my system by a wide margin.