The commonly used term 'Box Rise' is the sum of your subwoofer(s) impedance and the additional acoustic impedance presented by the enclosure and the vehicle. You may wire 4 subs to a 1.0 ohm load on your amplifier, play the system and find out that your “Box Rise” is actually 4.5 ohms. That 1000w amplifier is now pushing between 200w and 350w! That does not mean that your car is not loud-as-hell; you could be breaking crap off your neighbor’s wall.An informative experiment would be to test your box as it sits and then push it against the back of the trunk or cargo area…maybe 1” from the port opening and test it again. Your numbers will come out way different due to changing acoustic load presented by the new location. Moving the enclosure around the interior can greatly affect the SQ and SPL of your system.