I did some testing by moving my 12" sub around in the car. It's a cube box so it's pretty easy to move around. I did a base reference level with what I had at the time which was a 40hz sine wave. I set my SPL meter on the dash and looped the track.
With the sub in the far back and firing up=111dB
on rear deck and firing to the rear=111 (surprised me)
on rear deck facing forward=109
on rear deck facing up=108 (kind of disappointing)
in the back seat area facing up=106
in the back seat area facing forward=106
I'm going to try an experiment and have the sub in the back seat and facing up and then get a rigid PVC tube about 12" and have it go up and over the back seat to the rear. All the output will exit to the back of the car in front on the storage well. If the output is real close to the highest levels I got above then I can build somthing like this:
Due the the longer path length the bass may play even lower than in the far rear which would be a bonus. I'm real curious how it will work out.
Aside from the higher output I didn't notice too much difference in the overall SQ. Firing into the seat backs you can feel them, but as far as SQ is concerned, the differences seem pretty small. I didn't listen to much music in that configuration.
5dB is pretty big to me, that's like what, more than the difference between one and two subs? I'm going to try an harness that extra output if at all possible.
Funny thing. When I was doing those tests the output stayed at 111 even when I rolled the windows down (temporarily), so maybe the car wasn't being pressurized very much at that low SPL. Then, when I hit the latch release, the hatch pops up a little bit and the SPL drops to 105. That surprised me the most.