Mb quart and audiopipe also make 1k+ amps for $100-$150. Look at the reviews on this 1500W rms amp from audiopipe. http://www.amazon.com/AudioPipe-APCL...audiopipe+1000

Or this one Which might fit under your seat.
http://www.amazon.com/AUDIOPIPE-APSM...s=600+watt+amp

Have you thought about 2 subs? One sealed 8" sub with 12mm xmax doesn't produce much bass. I'd build a box for 2 8" subs(separate chambers). I'd buy one d4 sub to start. It'll wire at 2 ohms by itself and two of them will wire to 1 ohm. Then I'd buy an amp that produces at least 300w rms @2 ohms and 600w rms @1 ohm.