How tall are your ceilings? Are they flat and without obstruction? How far is the Arc from the ceiling? How far are you seated from the Arc?

The +/- changes the volume of the upfiring speakers relative to the other speakers.  It does not change the angle at which the speakers fire.  I also have my Arc mounted above the fireplace, and find that if you place a book or similar edge underneath the Arc in the back to tilt it forward a bit, the angle is improved and aims (after bouncing off ceiling) upfiring speakers at the seating area.