My 4B-ST lives in the bottom of a cabinet. However, the doors are open when it's running and the 4B sits up on some posts so that 1/4 of the available space is below the amp and the 3/4 of the remaining space is above it. The Bryston sits in an area that is around 18 inches tall.
When I'm running the amp for an extended period of time, I check the heat sink areas, where the transistors are mounted, with an infrared thermal thermometer. The highest temps I've ever recorded on the heat sinks were 128 degrees F.
Typically, the top of the chassis is around 112 degrees F after it's been running for 3 or 4 hours.
The Bryston is getting a lot of work, at the moment, as I'm ripping all of my CD's to the music server that I built. Any tracks that J River complains took re-reads to capture are played in their entirety. So far, I've only had to re-rip 2 tracks. Both on CDs that my kids borrowed. (I'd hate to see what my 18 year old would have done to vinyl (seeing how hard he is on CDs).
HsvHeelFan