Hello there!!!
You can! But I didn’t. If you go horizontal you’ll have to re-enforce your beams in the wall if they are load barring to avoid sagging and other unwanted structural horrors.
I went with the 7.5 for my middle channel and it is amazing :)! I installed it vertical too. It fills up the middle of the room and handles the deep sounds very well:)!
The 664s I was looking at also. Only problem if I’m correct they are 12” deep. My wall couldn’t go that deep.
In my opinion, having the cinema 7s vertical on the left and right are the way to go. You get the full array of sound coming at you. The speaker has a total of 5 built in. They are no deeper than 4”. These speakers can go in any wall no problem!
You may pay more toward the end, but sound will hv u never want to go anywhere else:)!
I hv one more cinema 7 to install on the right vertical.