If you point the ends down, yes it is going to be louder then if you point the ends out to the sides or the back. That is only if they are in the same location.
Anytime you angle your ends down, it's going to be louder. You can really notice it, if you drive over a concrete bridge. When pointed down the sound bounces off of the pavement and back at you.
If you let the ends come out in front of the rear wheels, closer to the cab, then that will be louder as well, to you the driver that is.