If he hasn't set his delays for the rear subs that could change everything.
Also, keep in mind that you can only do two things:
1) Make the golden seat perfect.
2) Make all the seats some-what ok.
The one exception to that rule might be a DBA alignment. Given how big those subs are relatively to the room I doubt you'll have much success doing that physically as-is.
The bad news is you are hitting up against the Iron law triangle.
Out of: price, space or SPL... you can only have two of them concurrently!
You want high SPL but are limited on space, which means the solution will be VERY costly.
You'd have to throw out your existing subs and go with at-least 4 sealed 24's and a XBS FP20k + miniDSP to have a fighting chance of high-spl in that room/space.
The only other solution is maybe switch from ported to sealed and add a BOAT LOAD more UM-18 and lots and lots of power (think XBS FP20k's...) That might even require a new 200-400amp service drop or pulling more/thicker wires to support the wattage.
Maybe you could add a bunch of B&C 21's and tune them to 30hz instead of 16hz, or make them sealed.
Alternatively adding 32 PA-460's in your room by basically building two of the dual-opposed sealed boxes I built for my MBM's, two in the front and two in the back. Powered with 2 FP20k's.
Yeah... there is literally no cheap way to get louder in your space!!!
The only thing that comes to mind is Quad Devastators maybe...
Adding powerful mains would help to a degree FOR-SURE.
I mean it literally looks like you have a 6db/oct 30hz LPF set in your inuke's. It looks like a house-curve but you are saying the DSP is OFF.
Do you have some funky AVR EQ enabled or what?
If you measure about 3inches away from the cone-middle it should be almost ruler flat to 200hz or whatever LPF you have set... (assuming you are measuring 1 sub at a time)
Something ain't fully right with this rig.
Are your subs summing 6db per pair like they should be or what?
You should be over 120db, if not 130db in that space with those subs... I would hazard a guess.
Definitely not no 90db...
Are you SURE you ain't a basshead?