I googled your amp, it seems that it is not rated for a 4 ohm load in bridged mono, only stable to 8 ohms.
Not so sure it would like driving the CSS SDX-15, but it would be worth a try. (http://www.creativesound.ca/details.php?model=SDX15
You could always wire both coils of the SDX-15 in series, and put the amp in stereo, use only one channel.... this would give you 300 watts to it.
That would eat up almost half of your budget there.... and if you need a subsonic filter, you could use a few different devices. Crown XTI with built in DSP.... ( blew your budget ) Behringer FBD, or a Behringer DCX 2496. ( with your existing amp )
Crown XTI 1000 if you can stretch the budget gives you flexible DSP, and 1400 watts into a 4 ohm load in mono. $ 500 at Parts Express, and less other places. ( fleaBay, etc other resellers )http://www.partsexpress.com/pe/showd...number=245-435