Speakers on top of subwoofer

I side with @soix on this. A speaker balanced on top of a sub is going to move. It may even fall off. This is avoided in 3-way systems by bolting, gluing, or otherwise fastening the sections together in the optimal configuration, which of course depends on the crossover. Which of these attachment methods should be used with an expensive speaker? I say, "None."

Then there’s the problem of stabilizing the sub on the floor, with an added weight on the top, raising the centre of gravity - it goes on. I wouldn’t touch it. YMMV.

not ideal and looks like a kludge but if you really enjoy the sound....

just make sure you have good iso dots under the speakers. 

Definitive pioneered combining loudspeakers and subs in the same cabinet for decades and Golden Ear is doing this as well. I haven't seen any complaints about them.

in the same cabinet

being the operative words.

I have enjoyed my Sony M7 sitting on top of my SVS SB16 ultra without any issues of vibration 

https://youtu.be/iiZ8xf2pDco?si=THxlHfGQ1wxOgDwe