Heat, weight and EMF coupling are your concerns with amps.
Hot air rises, so if in an enclosed space it’s probably best on top. If you have plenty of ventilation it doesn’t matter as much.
Then there’s the weight issue. If an earthquake or tipping over is at all a concern it should be on the bottom.
Lastly, that transformer (assuming a linear power supply) is going to put out a lot of EMF which can couple unexpectedly to other gear if too close to it. Don’t worry, you’ll 100% hear it if it happens. :) It’s not subtle.
In my case I use a 3 shelf Butcher Block Acoustics rack and have my amps on the middle, plenty stable, lots of ventilation.
If your amp is heavy and your rack is tall it really should go close to the ground.