Got a new Onkyo that's huge, It barely fits in the TV furnitue leaving only 7 mm room at the top for ventilation. I don't know how much heat this thing will take.
it will not explode but it might fry. After a few minutes of use, touch it and if its quite warm or too hot to touch, just set a fan up blowing air in.
Most receivers have thermal protection and will shut down if they overheat.