does enclosure location effect spl/sq?

recently, my friend installed a jl audio 10w6 in his trunk against his rear seat, later when we moved the enclosure towards the center of the trunk, we noticed the subwoofer sounded a lot "louder". now, i'm planning on purchasing the following enclosure for my car, but i'm concerned that the same issue will arise in my car as well. any input on this would be greatly appreciated

trial and error my friend, its really the only way to tell what will happen in your specific car with your specific setup.

You've made a good observation by moving your friend's enclosure and noticing the difference.  (Just like real estate...location, location, location.)  Keep this in mind as you design your box...allow for flexiblity of positioning.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.