Heat rises - so assuming the tube'd guitar box is the hottest of the bunch putting it at the bottom may not be the best idea. Look at each piece of gear - are the vents on the top, bottom, sides?
Is there a reason you need all four pieces in the rack case? You'll probably never need to touch the hard drive or power conditioner, apart from on-off. I'd leave those out, stick them on the desk or under it, whatever. Then leave some spaces between the interface, preamp, and top/bottom of rack case. Get some vented blank panels in there.