I would suggest going to a matched-directivity two-way speaker like the three π or four π loudspeaker. Add a pair of three π subs and you'll have deeper extension than any speaker without subs, plus you'll gain the benefit of muti-sub room mode smoothing. That is an excellent setup, one that I use myself.The thing is, it seems to me it would be easier to build from scratch than to cut out the baffle with a sawzall and replace it with one having the right size cutouts. But either way you go to get to the goal, that's the kind of system I would build. Uniform-directivity mains and subs.