Well, again (and I'm not being sarcastic) these units were installed by the guy who was the Synthesis® honcho until January of this year. He did the spec design and is on the patent for the S4Ai, did the sourcing of the Synthesis® electronics, and knows the systems inside out from soup to nuts.
He had a firm idea on how to stack them and in what order. I had a ton of 1U ventilated spacers on hand if needed. He stacked the amps one on top of the other without hesitation, with me occasionally getting my fingers pinched in the back.
We had a lot of space left in the second rack and could have spread it out if needed.
Originally, I planned on having forced ventilation from the closet to the outside, but he felt it wasn't necessary. After running them for several hours, I have to agree.
The S5160 and S800 both have fans, so they're self cooling. Space between is not needed, although there's no law that says you can't space them as far apart as you want. Even though the faceplates are touching, there's still an air gap between the chassis in back.
The S400 uses radiant cooling, hence no fan. That's why it's on top and why there's a ventilated spacer above it. Since it's currently not used in the system, it poses no heat generation issues anyway. When it's hooked up for 2nd zone use, it will only be on when the second zone is in play.