The short answer is No.
While it would help, your speakers to sound better due to reflections from closing the open-end, Since speakers only care about there physical locations, to your MLP and this is still a positive to do so. Sub/s wise, it will not change a thing. And the sub/s will still try to fill all open area. While it is not the answer you wanted to hear, I would still do-it just to improve your speakers sound signature
Darth