DD subs love large boxes and large port area. I had (well still have 1 left) 2 1508 supercharged in a 2.5 cu ft box after displacement (yes a little bigger then DD specs) tuned to 35 hz. And I was doing hi 139's and 140's on music on a DD m1b amp. I was fairly flat from 130-140 hz within 1.5 db's. SQ was amazing!! Personally I love the lows and wanted a low bass monster from 8's and that's what I got in my trunk car (Lexus is 300).
When u said SPL I doubt u meant u r planning on competing? If u want a loud daily driver I would recommend either 2 4's or 1 6" aero port tuned between 36-40 hz and build ur box between 2.0-2.5 cu ft after displacement. Don't forget the precision port calculator doesn't subtract for driver and port displacement. Basically if they don't sound very impressive something is wrong.