you would more than likely hit more spl with the wall.....cutting the distance in half raises your spl by 3 db .....i put that to the test with my spl meter in my apartment and it was true
i believe its like if youve ever noticed if you stand in a corner the bass seems louder. so im guessing you would want to have the most bass build up at mic placement. or if your not competing than head placement lol.
corner loading is when you place the subwoofer in the corner of a room or in this cas a trunk wall and the bass gets reinforced by the wall itself its usually place close to a wall....within inches
there is another technic called slot loading and that is using two subwoofers facing each other a specfied distance apart and it has to do with the waves compressing and releasing for a greater spl effect
and you have to have exact power going towards each sub or one will throw the other sub out of wack.....from what i have been told atleast.....the guys i asked about it said you can blow the subs doing it but if you get it just right the outcome is ungodly loud!