Check out MTX JackHammer 12" subs ($90x2) , and the MTX Terminator TN200/2 mono amp ($130) (or go 4 channel if you want). I hooked this up for my friend over the summer after he bought them off a kid for $200 along with a Kenwood headunit. I don't know much about amp and speaker brands, but these things are out of fucking control. Car alarms go off without effort. So if you have $400 to spend you could get that setup plus an enclosure than maybe spend some extra money on speakers.
I have 2 10" xtant subs and a kenwood amp that i got for free, it does the job pretty good.
I recommend installing it yourself if you have any knowledge of wiring, even though you don't need to know much to hook the amp up (power wire from the battery, ground somewhere in the trunk, run RCAs from the radio and a remote wire from the radio... not too hard.)