Originally Posted by Goofyguy
Time alignment is the true answer. Taking the Woofer out of phase can have other acoustic effects that you may or may not hear. The proper way is to either have proper time alignment, or remove the underseats from the equation.
Again this may work for some frequencies but not all.
Now that I have a PXA-H800 (replacing an MS-8) I can change phase and T/A on the fly, and my results supports your post: changing phase will give an apparent bump in SPL but with a detriment in SQ. The sub impact is simply gone, midbass is up and some audio artifacts -for lack of a better word- are heard all over the place. Like something is... out of phase
T/A is much more effective, although too much of it will make the sound to be like in a tunnel.