The Kappalite 3012LF is a wonderful FOH/PA driver. It is not IMHO suited for bass guitar. If you choose to load a cabinet with an additional driver of different size to enhance/add midrange you are venturing in to a place of diminishing returns. Choose your driver carefully for it's application and voicing with sufficient bandwidth (and there are many available) and tune your cabinet carefully (if you are porting) If you want more "woof" use a rectangular vent, more hifi..a duct. If you choose to mix drivers you must consider sensitivity. If you 15 or 12 is 98db your 5,6, or 8 should be the same. Then you need to either electronically align (as EAW, JBL and Meyers began doing in the late 70s early 80s when they were building boxes with mixed drivers) or mechanically align them in your cabinet with the backs of the voice coils aligned. In the 2000s we're back to where we were in the 60s and rightfully so. Line Array. Again, just this old musician/engineer/designer/builder's humble opinion.