@donavabdear
This is a simple observation by Floyd but it has been a MASSIVE benefit in my room.
An in-ceiling loudspeaker is used as the Voice of God. Others could have replaced some or all of the elevation speakers. But, knowing that the direct sound has a dominant effect on timbre/sound quality I decided not to compromise, and used high quality bookshelf loudspeakers in custom mounts, aiming them at the prime listening location as shown in the following floor plan.
Placing the matching bookshelf speakers at front and rear heights directly above their bed channel counter parts matches timber as Floyd states, they also better time align with the bed channels and in my experience are incredible at pressurizing a room. A perfect way to demo this is get a copy of Kraftwerks The Catalogue concert bluray which has an Atmos mix and can be played in 3D or 2D (3D is AMAZING if you can wangle it but the main thing is the atmos mix). Talk about pressurizing a room at reference volume levels, it will blow your mind. The bookshelfs as height channels have no trouble keeping up, the smaller in ceiling speakers I just don’t see having enough muscle. My heights are active and so the muscle they add to a concert or action movie are OTT. You can achieve the same thing with passive bookshelfs like Floyd did too: