Amps on top?

That one is new to me. In 50 years of pursuing the high end, most of the time my amps weighed well over 100 pounds and went on the floor.
 

If you are lucky enough to have mono blocks they definitely sound best placed within a foot of their speakers with ultra short speaker cables. Virtually all amp stands are low to the ground intending to be within a couple inches of the floor. 

I could see someone say that because they tend to be the heat generating component. But not anyone familiar with the high end audio industry.

Never heard anything about the shelf location - have read about having x inches clearance for airflow.

My Amp is on top.   Tube amps need ventilation.   If it was really heavy I would get an amp stand and put it on the floor.   

My Tube Integrated is on top, high enough to receive remote beam to control volume, nothing above, have heat rise directly to the room, not too close to the side of anything else. And, it’s a beauty to see.

When I had amps with no remote volume, SS McIntosh, and 30 wpc tube mono blocks, they were on the bottom shelf which was tall and wide, full open front and back. BAD location, I got a torn meniscus lifting the 80 lb McIntosh amp out of there to sell it.

I depends on system configuration.

Integrated amp on top so you can read the dials and adjust the knobs.

or

Power amp on the bottom (or a dedicated stand) because it's heavy and to make the speaker cables shorter. It doesn't have anything to read or adjust.