Re: How much of a difference does airspace above the box make
If you are facing the subs toward the rear seat then you should put the box as close to the taillights as possible. That is unless you're going to seal the box off from the trunk and into the cabin. If you don't seal it like a wall into the cabin, the more airspace you have around the box you'll get more cancellation, i.e. it won't be as loud.
If you were facing the subs toward the rear, then the more space around the box the better. It gives the sound more room to travel into the cabin.
I have an oldschool Cadi and have that same hump problem in mine. I don't even waste my time with subs larger than 12s unless its subs up/port back or side because of the clearance issue you have also
Refs: Korn4Life, Seth3515, JusBloud, mgeagles, Rashaad, Dacheatham, nuboot, Silentbass, Lysfys, Woofercooker, Redgp, Boominspl, Cameron B, Beaver1992 x2, Whiteram,Juukno, Towncar, Cravingbass123, slam330, rgplayer, TeamSouthrnBass, Anniku, Goose5741, KyngHype, Southsyde, HRTBRK1, mbrooky, jonny101abc, richardparker