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.
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.