mtbdudex on the Cult of the Infinitely Baffled forum had put together a good line array subwoofer using 4 of the IB15’s in his home theater. His thread on the Cult forum is extremely detailed showing all the planning and build stages extremely well. Many in depth pictures of the process.
This is what it looks like in the room from one of the pictures he has up:
Follow the following link to the Cult forum to read the full thread and see pictures of the build process: