Murph's points are well-taken. It's not so much that I'm recommending against it if you have no alternative; it's that when Axiom designs and measures its subwoofers, they are designed to work sitting on a floor in "free space" and that is how they're measured as well. The Axiom sub has its own carefully tuned enclosure. Putting it inside another enclosure introduces a completely unpredictable element.
Your particular room, the sub's location in the room, and the listening area/couch/seats' locations are all elements that combine to determine the quality and quantity of deep bass you'll receive (in addition to the subwoofer's intrinsic performance). It's difficult enough getting all those factors working in sync without further complicating matters by adding another unknown variable--putting the sub inside another enclosure.