There is no valid technical reason not to stack them - at least in the case (pun intended!) of NCore.
Hypex people seem very confident with their product and say even a minimal distance between amps and PS is acceptable.
Practice seems to indicate they are right (e.g. look up StigErik's NCore layout and his loudspeaker efficiency over @ diyaudio).
Regnad's enclosure was part of a group buy we organised - not because I'm a good person (
) but because I needed this style in specific dimensions and SiliconRay wanted a minimum of 15-20 enclosures to make a custom batch. We ended up buying 30 enclosures in total and I must say the pics don't do them justice. They're all sold out but if you get demand for ~15 enclosures I'm sure they would make another batch.
You'll find more about the enclosure and possible layouts we considered here
My plan is to start with a monoblock setup but the final setup will feature 3xNC400 + 2xSMPSs per enclosure (powering active loudspeakers).