The HDMI board certainly produces more heat than I thought. My 3008 would get quite warm/hot especially at the upper-rear-left corner - even when idle. When is use, it wouldn't take long to get the temperature high enough to trigger the cooling fans. This was down to the speaker load and only having 5" clearance above it. Using some fans solved the problem.
I replaced the 3008 with a 5508 and UPA-7. The 5508 still got pretty warm/hot, but the UPA-7 also got fairly warm. This also suggested that the speaker load was a bit of a tricky one, as other people were saying that their UPA-7's remain relatively cool.
The 5509 is pretty similar in heat to the 5508. It is not easy for me to provide more than 5" clearance, so I'll continue to use the fans I bought for the 3008.