Emax

I know from the sticker that I'm not supposed to cover the vents of
my Emax, but these were meant to be racked after all. Do I need to
keep a space open above the Emax or will it breathe well enough racked
close?
    I don't have a lot of space right now and 1 slot is enough to put
a crimp in my ergonomics workflow, but I would hate to hurt the emu. 
Has anybody had problems with overheating?
I have a 6 unit desktop rack with an emax on the bottom, ensoniq dp4+
above it, power conditioner up above and a mixer  on top.  It does
warm up a bit (not yet egg frying time), but how hot is too hot?