I use these Kappa 12A for midbass in my 35L front ported tops with 3500hz filter.
I have the B&C DE16 as tweeter in a ME45 horn.
I am very happy with the choise of the Kappa 12A, because they are extremely responsive, match my tweeters, have a clean sound and can take alot of power.
If i dont use my subs i have to use quite alot of EQ to get the "correct" sound for bass and drums etc...
But with enough power these will for sure suprise you with the bass capacity!
I have 4 tops and 2 custom built 18" horns. Its a perfect match for the rest of my system.
If you are looking to build a true subwoofer with Kappa, my thougt is that the LF or maybe PRO verision would be a "better" choise.
Overall great speaker, good sound and quality. And thanks to Thomann as usual with the great shipping and support.