The sub changes its sound when you rotate it because you are changing the distance from sub to wall. Keep in mind it is the cone's position that matters, not the center of the enclosure, so rotating it does not keep the distance equal.
Even slight changes in sub location can make an enormous difference and experimentation is key. Try putting the sub in your listening chair (and up a couple of feet to approximate where your head would be), then move your ears to some potential sub locations. The spot where the low frequencies are deep yet smooth is the best starting position for the sub. You will note that the sound gets louder (though not necessarily better) when your head is right against the wall, floor, ceiling or exactly in the middle of the room.