Interesting. When I had M-80 I had it set just like you said, but with my current setup I put the amp on top. I don't see any negative impact honestly, but I will measure the temps once I have a longer session. I am not too worried about it though, M-80 is class AB and Hypex Nilai is D. It doesn't really get that hot.I don't know if it matters but Onkyo told me specifically not to put the M-80 on top of the P-80. The P-80 always goes on top. Not sure if it's because of the heat the M-80 gives off or of the P-80 needs the airflow. Just wanted to give you a heads up.
What he said is other way around. M-80 is power amp and P-80 is pre-amp. Onkyo put pre-amp on top on every promotional picture. But my intuition tells me otherwise to be honest, I want more airflow for amp...True of any pre-power, if you're going to stack them always put the power amp on top.