Thanks, Phil. The Middle Atlantic Racks let the height be set to almost an infinite degree, because each shelf can be attached at different positions within the rack. The 4B probably wouldn't get smoking hot, but the 9B, with all five channels going, would probably get up there.
I noticed that Bryston sells fan units to help keep things cool. James, care to comment on how they work, what they look like, and what they might cost?
It may also be possible to run my air exchanger vent right down to the bottom of the rack encloser, and have air flow coming out through that.