for anyone with subs. i need some help

[C-10volvo]

i installed a new jbl 1200 watt amp not too long ago. it all works and sounds great for the most part. the real problem i have is the low range. its really weird!! when the doors are shut, and the windows up my spl reaches 132. but when the windows or down or the doors are open it reaches 139. the box is sealed well, and the subs can handle the power. could this be a drawback of having so much power in a small cab?
If you have subs in your truck, could you preform a little test for me? turn it all on, and crank it. open a door and listen for a change in low and mid bass. ive ran out on ideas on what this could be. a change of spl from 132 to 139 is a lot! thanks for your help, ken