The way I implement such a system is to put a subwoofer below or near a three or four π loudspeaker. I know this is something you're familiar with. Having them separate lets the user have some configurability. That way you can put the subwoofer off to the side a foot, and the mains on a short stand, so the midwoofer is in a different location in all three planes. Overlap them through the bass range for best smoothing.

Another way I implement something similar is in the π cornerhorns. The woofer is low enough to the ground that it doesn't suffer floor bounce. The midhorn doesn't suffer floor bounce because of blending with the woofer. Its response is low enough to smooth some of the higher modal range. Subs can also be added to this configuration to further smooth the lower modal range.