If I were you, I would stick to a pair of 816. You will have horns working down to 180Hz. Then mid bass down to 60-80Hz from the 816. Which you can complement with your subs.
Also, the woofer in the 816 is about 2" deep and can easily be physically aligned with the compressor in your Mantarays, with the Mantarays' face portruding by probably only 9"-12"
It makes way more sense to go with those 816. I won't add anymore to the discussion.
Lee