@OP The Pass is a much better sounding amp than the Mac and I hope you will be very happy with it. Most of the heat it generates is via the heatsinks but you really should have a couple of inches at least of air circulation space above the top plate. So it would be better on the top of your rack. Unless your rack is not level, the amp the load of the amp is compression only so there are really no concerns there.
Mac MA 352 vs Pass INT-60
I've had an order in for the Mac MA 352 for 3 months and supposedly it's coming shortly. I wanted the tube front end but now I've got an option since I no longer think I need a balance control and I just found a really good deal for a new Pass INT-60. I'm running these, hopefully, into KEF R7 Metas at 88 dB. I figure the Pass is adequate into those 4 ohm speakers in my 25x14 space. I also figure the Pass has as much tube-like quality with likely a better high end than the Mac. If price is not a factor, what do you think? The Pass is a beast, but I can deal with that, and my first real amp was a Threshold 400A. I have some worry about current delays in Mac serviceability also. :)