Never heard of Resonant Engineering. Took a quick peek online, they look decent.
I'm just wondering if your looking for SPL, SQ, or a blend. Probably a blend.
15'* aren't going to have that "tight" sound. Physics don't allow it. The mass of the cone alone is enough, yet folks are looking for stupid power handling. That USUALLY means a big coil with lotsa copper, aluminum, whatever they're using for the conductor on the coil (I would not want a speaker designers job these days). Gotta dissipate that heat.
The suspension compliance can help alot at the loss of efficiency. Enter the motor.
I haven't seen the Dumax on these subs , so I dunno about the BL thru the gap.
I do know that listening to this ONE 12" is very gratifying. I personally wouldn't go with anything other than sealed if you are looking for SQ. That 15" in a ported or bandpass could probably do structural damage to your car with enough power
I'd recommend one 12" for SQ. Two if you're looking for stupid output with a bit of SQ.
Obviously these are my opinions.
Whatever you decide, let me know, maybe I could give you a bit of help designing the box.
I've got some decent software for modeling such things.