Not sure what calculation you did which told you "sealed boxes only" but the titanic does fine ported and doesn't need that big of a box. If you don't want ported, no worries they only deliver twice the spl all things considered equal. Food for thought: There is a reason no commercial theater or IMAX you've ever seen uses sealed subs(that I'm aware of). It takes an army of sealed 15s to displace enough air to work a space the size of yours. As others have said 4 sealed 15s are a start but nowhere close to reference.
As previously stated(by another member) your box is currently almost twice the size it needs to be. I cant model on this computer but from memory the titanics in 3ft cubed nets about a .7Q. You could fit two more drivers in the boxes you already have and do dual opposed if your bracing permits. You will be very slightly on the small size but it will help protect you from excursion problems and driver damage.
Also, how do you know your not seeing x-max? You might be there and not even know it. Distortion is more difficult to hear from a sub and unless its complaining loudly mechanically you prob wouldn't even know. Just sayin'
Corner loading is good for spl not so good for a flat response unless you load all four in a sealed rectangular room. I agree though if you need some bump corners are free spl and worth using if the system runs out of steam.