Do you know how high your Cathedral ceiling is, at the wall and at the peak?
Just some quick number crunching, assuming a 4' peak and 8' walls would put your room volume at near 5000'3! And you say its open to other space, as well.
This is a good read:
This article outlines our test procedure in determining a subwoofers room size recommendation based on its max CEA SPL output while making room size and subwoofer distance assumptions.
www.audioholics.com
Now I know you don't have to Pressurize the full space, and creative placement can go a long way in helping you out. But... consider that you do need to look at something with more firepower!
Buy the best single sub you can allow your self to afford, now. Then plan for, and start saving for a second.
If you do it right, you won't need to replace your Subs for a much longer time.