A.J.
Hey remember that sound is movement of air. If those boxes are air tight, most of the energy is being directed forward. Vibration through the wood is where most energy under the stage would come from. Make sure you not only cross over the low to mids, but also take out energy on the low (20hz) side as well. The manufacture is recommending a 12db butterworth HP at 25hz. The speaker is rated to 28hz at (I assume) 3db down. Not sure what kind of music you have going on, but I'm pretty sure you don't need it that low.
I would move that up a little and see how it responds. Of course I assume that you are aligned to your mains. You have very large 18" drivers (2 per) that are hard to get moving and once they are moving are hard to stop because of all the mass. Anything you can do to minimize the travel might help them tighten up. The best way I know how to fix that is to limit the low end of the lows.
If you are not using Aux fed subs you might try that. Anything you can do to eliminate all material other than lows you want to hear will help.
If you have Smaart or something similar, an impulse measurement might reveal if there are room issues involved.
Is it muddy on everything or the peaks like a kick drum? I call it muddy when the bass player moves a note one step and I can't tell the different or hear the transition.
Just curious, you ever have problems with video while the subs are playing??