The extremely low frequencies that a subwoofer is active in are omnidirectional, so the direction the driver is pointed isn't relevant. Only the location of the driver matters. And in a small square room without bass trapping, I think there's really no way to get flat response from a subwoofer.
Bass trapping helps quite a bit. The more bass you absorb, the less bounces off walls and is available to either double the output (at certain frequencies and locations) or cancel the output, (at certain other frequencies and locations). In theory, if you had total bass absorption, the frequency response would be the same in every location inside the room. Just as it would be outside in an open field.
I've got bass traps in every corner possible, and I've stuffed dense cotton above all of the drop-ceiling tiles. The improvement was substantial, both measured with a meter, and in listening.
Without bass trapping, you can move the sub around, which will change the specific locations of the various peaks and nulls in the subwoofer/room response. So, let's say you're able to get 30hz at the correct level right at your ears. A few feet away, 40hz could have a null. Move the sub to a new location, and at your ears, 30hz could now have a null and 40hz could be perfect. No matter where you put the sub, both 30hz and 40hz will never be the same at any one location.
