40mm does not sound like a lot since the majority of the vent slots are on the top of the 3808.

I can tell you that my denon 3808 does not get that hot, only slightly warm to the touch, when driving the m80's throughout a movie or a cd... but I have it on the top shelf that is open on all sides.

I would suggest following whatever instructions there are in the user manual for clearance regardless of what the impedance is of your speakers.

-David