I would imagine putting the WAP in the middle of the house on the second floor, on the floor, would give you the best reception. Think of it like this, your WAP has a ball shaped range. Your house is like a box ... put the center of the ball (the WAP) in the center of the box.
If you have it on a wall, you're covering outside the wall where it doesn't need to cover.
And use 91.47 ... 91.31 and 91.33 have given me problems.