in my civic hatchy back in the day...
my 2 rockford punch HE subs in a 10' matched box with a 20' angle pointed to the rear ~1 foot from the hatch was the best sound.
in the mirage, the same box with pentagon xplods is tons clearer (i prefer clarity over bump) pointing forward. the xplods outpound the HE's but have a bit less range.
it all depends on what you like. like said about try it how you like it.
if you have a nice stereo, you can do imaging and phasing. my alpine does all that shit. you can do some "calculations" step by step to set the imaging for all four standard speakers, and reverse the sub... turn down the power output to it and adjust the bass for the sub and standards respectively. i also love the motorized 3-angle tilt face, green/amber buttons and nice easily readable line of text.