What would happen to the sound if my enclosure has a smaller volume than what my sub requires. Its only a 0.09 cu.ft. difference. For reference the sub is a MA Audio MA112Xi and the enclosure is a SubThump fiberglass enclosure.

 

You can stick in polyfil to simulate a bit more airspace. Might just give you what you need. After putting it in my two boxes, I can notice a difference.