if you can go larger, the dayton 18" for $250 in a ported 8 cubic foot cabinet tuned to 20hz will do about the same performance with 1000 watts on it.
the stereo integrity 18" in a ported 6 cubic foot cab tuned to 20hz will do a hair less with 1000 watts on it.
the si subs can be ordered in dual 4 ohm, which is nice because they can be wired to net 2 ohms.
many cheap pro amps such as the inuke3000dsp output max power per channel at 2 ohms or 2 drivers bridged.
the amp is $350 and outputs a real 1100 watts per channel into 2 ohms.
so ofr $600, you can get started and for another $250 go for another one. :-)