We would generally recommend against stacking Hi-Fi devices on top of each other.
If you do have to stack units on top of each other, we would suggest placing the CXA61/81 amplifier above other non-amplifier products, as placing the amp on the bottom could lead to overheating.
As the CXA amplifiers do not use an internal cooling fan, the cooling of our CX units is entirely passive. So, the more 'breathing room' you can provide around the CXA's venting space, the better. Ideally, air should be free to flow from the top of the unit without any obstructions, otherwise you can run the risk of overheating the amplifier.
The ideal breathing space is around 4", but we would recommend leaving as much room around the unit as possible.