That depends on how low the midrange horn you are using can go. The K-55 can be used down to 400 hz on the right horn. I am not aware of a horn now available that fits in a Cornwall size cabinet and goes down to 400. I know of a couple that will make it down to 500 hz all right.
Now if you are building this with the midrange horn outside the bass cabinet, you could use the K-400 or K-401 and cross at 400 hz. with a 7 uF cap.
Bob Crites
I am useing a selenium horn that gos down to 400hz and i am building the horns out side of the bass bin my bass bin is going to be the volume of the cornwall 3 witch is 6.607 ft3