Umm... RTFM... Allow at least 6 inches (15.24cm) above the top...
"Allow at least 6 inches (15.24cm) above the top, 2 inches (5.08cm) below the bottom, 3 inches (7.62cm) behind the rear panel and 2 inches (5.08cm) on eachside of the Power Amplifier, so that airflow is notobstructed. Allow 2-1/2 inches (6.35 cm) in front ofthe mounting1 panel for clearance. Be sure to cut out a ventilation hole in the mounting shelf according to the dimensions in the drawing."
Refer to drawing for further details...