Ok. first off, don't use a capacitor. To power your hifonics 2006d, you'll at least need to do the big 3 upgrade. There is a tutorial in the electrical section of this forum. You'll want to run 0 gauge wire to the amp, and you might want to consider a second battery like an optima yellow top in the trunk. About ported boxes, search for kicker 15 L7 in the enclosure design section of this forum. Read about what size and tuning other members used for their 15 L7's and what they thought it sounded like. Don't buy a ported prefab box either, make it yourself or if your not into that find someone on this site that builds boxes. If you've got the electrical to run your amp, and with a proper hand built ported box, your subs should have the potential to get extremely loud. Just make sure your read the tutorial on setting amplifier gain so you don't blow the L7's. good luck