Be careful with in walls. The commercial offerings are produced with little to no thought about how to go about an in wall system. Nearly all are open backed. Atlantic technology offer a back box.
Putting open backed speakers in a wall space is a very bad idea. The drivers are not loaded, and even worse the manufacturers are just throwing in their bass/mids designed for ported application. The offerings by and large are a cynical mess to snag the unwary.
Also you really don't want open backed drivers blasting away in a wall space. This allows sound to penetrate all widely in the house.
At the request of my wife I have just designed and built an in wall system. The design challenges are enormous. I have untold design hours in the system.
For a good system you need to make a false wall. Using an extension to a fireplace breast is one viable solution and one I used. You do need more room that offered by a 2 X 4 stud wall space, but not by a lot.
You need to select drivers optimized for sealed application.
If you want a sub, then I chose to use a transmission line design, as they are potent and lend themselves to enclosures with little depth.
This system is in a very large space like yours.
Here are the pictures.
The framing stage.
View with grills off.
Finished except for frames around the equipment cases. The parts will be back from the powder shop in a day or two.
This is a very potent and excellent sounding system. The units are powerful with large motor system with edge wound wire. Total power available and able to be handled is 1000 watts all channels driven.
This is not just good for an in wall system but comparable to very high end separates. My wife and I have had a chance to listen to it extensively now and this is an excellent system that has no equivalent in the commercial offerings. It can take anything that content makers that can throw at it, as well as being an excellent music system with great fidelity and huge dynamic range.
The speaker units are preserved in the C & C shop computer. I could make them available for you if you want a no compromise in wall system. The left and right speakers could obviously also be used as surrounds and rear backs as well as L & R in a 7.1 system.