Adding a sub will improve movies but... I think the rear corner positioning will be problematic.
The old saying goes that bass is non-directional, but there are limits to that IME. You'd have to cross over much lower than 80hz to not hear the sub in the rear corner, and even then, if you managed to get satisfactory results at even 40hz, you might still hear the bass note in the ear nearest the sub before your other ear making it sound unnatural or even unpleasant.
If you can accommodate two subs, one in each rear corner, it may work better, or could you accommodate it in a more central position at the back of the room?
If you are limited to that single rear corner position and your MLP is closer to it than the front speakers, you may be better off staying as you are.