Amps on top?

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.

I just had a new stereo stand fabricated by an off road truck builder I know.  Welded steel.  I bounced on top of it to verify it's rigidity.  I plan to put my new (larger) amp on top.  It is a tube amp and I want it to breathe.  Instead of shelves, I use planks that cross under each set of feet.   The amp is a tube amp and I want nothing above it. 

Under the main  amp I plan to put my center speaker and my power supply.

Under the left side (choke chassis) I plan to put my DAC.  Under the right side, I plan to put my streamer.  My external crossovers will be on the outboard ends.

Setting amps on the floor just makes them hard to deal with.  Mine will weigh 150 lbs in 3 chasses.

Jerry

Steel tends to sing (vibrations at a particular frequency). Its a thing so hopefully your off road truck friend was able to mitigate that during construction. Best.

This high end  fellow I know has a huge old heavy very thick solid cherry bookcase cabinet modified for a large screen in center, four monoblocks in it at bottom center.  A pre pro, two Richard grey power centers there 220 volt. At the very top left and right hand corners about 7' off floor are two monoblocks German high end amps 220 volt I believe or two cords on each. Before this he had his arc 750's up there. Also a high end turntable was in there. His ancillary gear is in a rack on left of room.  No room treatments whatsoever. 18 by 18' 12' ceiling.  Two huge JL audio subs sit next to cabinet, two huge base towers and tweeter towers out front. He had 5 dacs, the best being  a Berkeley ref 3.  Damndest thing you ever seen.  

Is there any other reason appart from weight and speaker cable length?