Welcome to LMS coil world. Put some 8" 's in the doors crossed @ 50 and be done with it. I've ran LMS woofers and had similar experience...then again @ 20hz there's no comparison.
Also those Korean high current amps mostly put out max power @ .5 ohm which doesn't really go with "SQ". That being said Sundown 2500 is a monster for it's size and the DD M3 is known to do well over rated (though I suspect either would be at low impedence... Either way you'll want a couple additional batteries to run any true 3KW amp properly.
All that being said I have a Stetsom 4K2d for 650+ shipping. Makes rated power @ 2 ohm and is very small.
*edit* Now that I'm remembering I was able to get a bit more output with mine by just moving the box facing a different direction. You may be able to rebuild your box or just aim it differently and change the response enough to suit you. More info on the box and vehicle may get some suggestions as well.
