What is the "Rule of thumb" regarding a 12" xcon and a 15" xcon? Say both subs are tuned to 32Hz and are in their "optimal" ported enclosure. Should you expect the 15" to be twice as loud? We tlaking 3dB, 6dB, or even 10dB? I am facing this same dilemma where my old accord can easily fit a single ported 15" xcon, but in a year or so I plan on getting a new car with a smaller trunk. The new cars trunk will easily fit the 12", but the 15" will be a bit tighter fit and I am debating if it will be worth it.