Component Stacking Question

Temp is OK.

Weight, maybe. Is the preamp strong enough for the weight?

Noise - Power transformers can radiate a lot of EMF noise, which can be picked up by other equipment too close to it. You'll hear it as hum.

Stacking the amp on top is the only way if you must stack. Better off putting each component on a separate shelf, with as much room in between as possible to reduce effects of EMI and restricting airflow. Typically, the power amp needs the most breathing space, followed by the preamp and then tertiary components. Remember, heat is the enemy of electronics. 

these two specific pieces:

I would put a 1" spacer in between them so the pre-amp rear vent could do it's job, then stack away.

Then how do you explain how an integrated amp works? It's all in one chassis in even closer proximity to the low level preamp section, and seemingly sounds ok....I see absolutely no issue in stacking.