....I used 608 for almost 2 months, no overheating problem. I have only 10cm gap above for ventilation, glass hifi shelf.
Once I discover very hot problem under "HDMI through" setting (so I don't need to turn amp on while can still receive HDMI picture and sound signal from Cable TV through to TV) rather than "Standby". It uses up a lot of power while being standby, and the top gets very hot. So I never set to "HDMI through" again.............