Just an idea, but have you considered just placing a thicker GIK trap directly behind your speakers, just on the floor, a couple inches from the wall? Take two from somewhere else and try it.
Leaving a gap between the wall and the trap will allow the trap to better deaden lower frequencies (and actually all frequencies). This also works with corner traps, btw, which are also an excellent idea.
Another idea is to put a slatted dispersion panel (typically hardwood slats on rubber) on the front wall behind the speakers and also the thicker trap in front of that. I did this in a literal bomb shelter and it worked wonders.