stacking components without leaving marks

Every shelf in my main audio rack has a pair or even three componets stacked. There is NO reason not to stack components IF they:
Do not radiate: too much heat or too much EMR.(electromagnetic radiation)
There are no other reasons, other than looks.
Usually tube amplifiers and tube integrated are the too much heat. Class "A" running solid state amps too.
Everything else is fine. I tend to keep the Phono away from the Digital, but that is just me.
Added: No stacking was invented by the folks who make or sell component racks for a living

Felt pads for furniture works. But I do like Lizs comment.

BTW... Don't recording studios stack most of the equipment.

Elizabeth, Not true. When you stack components you are mass loading the component on the bottom which will affect the sound.

Felt pads for furniture works. But I do like Lizs comment.

BTW... Don't recording studios stack most of the equipment.

Yeah, mass loading, so i use the component on top instead of spending another few hundred to buy some fancy gizmo claiming to be the best for mass loading, blah blah blah..
Though i own several VPI bricks I got dirt cheap (as in free, and $15 each for two more...)
Patio bricks are good for mass loading. Just glue some card to the bottom so they do not scratch up your stuff.
I loved using a zigzag shaped brick on my prior amp. A Forte 4a.
I use round (with a little crescent cut out) 5" diameter 3" tall patio bricks under my turntables. Eight patio bricks help keep the whole rack stable.