Just wanted to share my experience on center speaker placement.

The top of my TV is 5 ft tall while our listening position based on our seating is around 3.5 feet. The center speaker is level on top of the TV. The sounds seem muffled and hollow. I checked my connections and they are all proper and tight. Increasing the sound level by 1 to 2 db did not change anything. I decided to tilt the center speaker forward such that the drivers are aimed at the listening positions. This corrected the hollow and muffled sound.

FYI