With the power you have on hand I would say sundown sa15's without a doubt. I am running 2 of them in 8 cubic ft tuned at 33 hertz running off a crappy audiopipe apsm2000 in 2 ohm. At that load the amp is putting out 800-900 watts and they are very loud my friends find it uncomfortable at max volume..
If you are willing to go used I saw a pair of the sundown night shade 15's on
eBay still under warranty for 500 for the pair. Good thing about sundown is the 2 year transferable warranty.. He'll I had a pair of their e series 15's and was running the ap on them for a while in 1 ohm and they never missed a lick.
The image dynamics idmax subs are like 350$ each and are the shiz as well.