Stacking Components

Depends on what you stack and the order of the stack. High power amps should not be stacked and should be by themselves. Other low heat components can be stacked without worry. 

Very bad idea isolation is the way to go.You can never get this by stacking.

Not the best idea but can do if space is a factor.
I would put the heavier unit on top.

This all sounds academic. But what does it really mean, here in the real world? 

Many years ago back when I was just starting to learn how much these things matter I brought a set of BDR Cones over to show a friend how great they are. They had by this time been used under scores of different components in dozens of different systems, including ones just like his mid-fi rack of Sony, Kenwood, JVC, etc. It was always easy to hear the same improvement every time.

The only difference with his system, all his stuff was stacked one on top of the other. 

In this case he heard nothing and I could barely hear any difference at all. This had never happened before.

So the real world implications of stacking are you wind up with smeared distorted garbled dreck, bad enough to mask even a significant upgrade. You are in other words throwing your money away. Own goal. Don't do it.

Agree with what everyone else is saying plus the RFI/EMI interplay potential. You really need to give your gear a chance to sound their best and it’s you’re reducing that possibility by stacking.