As a former installer of pro audio systems, everything from restaurants to churches to arenas - IMO you're over-engineering this. You don't have to do anything fancy with the HVAC. Just cool the room as you would any other, although it certainly can't hurt to arrange the ducts as Middle Atlantic suggests, especially if you have the luxury of designing the room from the ground up.
If you're using an open-frame rack, then heat build-up is not an issue. A typical method of ventilating an enclosed rack is to put a couple of 4" muffin exhaust fans at the top, to pull out hot air as it rises. Naturally, this works best if the rack is sealed and has vents at the bottom for cool make-up air. Another trick is to put all the heat-generating gear like the amps at the top of the rack.
Wayne A. Pflughaupt