I have a BHA-1 on top of my BDA-2, on top of my BDP-2. All of that is in a cabinet that is enclosed with a door on the front. The back is open, and I added a 120mm computer fan that I toned down to run at only about 600 rpm. It's completely inaudible.
Without that fan, the BHA-1 predictably overheats within a short time of use. With that fan and very minor air circulation, the whole thing never gets more than warm to the touch. I have similar situations for my Bryston amps, processor, and other equipment. That very minor airflow makes an enormous difference.