rack mount ?

Frankly I'm not sure how much difference it will make. Heat normally rises - and this heat will affect the temp of the components above it. You really want to break up the column of heat from the amps and a using fans directing heat in a horizontal flow can be effective.

The question may be 'do you really need' this fan in your set up at all. One uses a fan to draw cool(er) air over a component, or one uses a fan to cool an enclosure by increasing circulation and drawing in fresh air. I think if you are using an open rack that cooling may be unnecessary, especially if you allow some space between components to allow for the natural circulation caused by the natural rise of heated air.

Put it where it is needed the most.

I would put it under the power amp only. You want the coolest air feed the highest power component, and you don't want hot air leaving the power amp feeding the low voltage preamps.

So I would place the preamp/selector switch at the bottom, followed by a space, then the fan, then the power amp on top. The preamps and switch don't need cooling.