Can you experiment with placement now in an existing room? Come as close to the couch/wall position for the media room as possible. Make temp mounts, cheap lumber works good, and try the 300s in various physically acceptable positions.

Maybe when building the media room you could add mounting blocks between the studs in multiple locations that you might decide to use?

Perfect positioning is all well and good but the bottom line is what is possible and how it sounds to your ears.

My surrounds Play 1s recently upgraded to Era-100s are horribly positioned. Too far back, too high, not centered on the listeners and pointed the wrong directions. Guess what, they sound better than no surrounds. That was the spouse’s other mounting option so I’ll take what I have and be happy with it.

I’m going to have to make a decision before I move in.  The power points will be in location for the speakers so it’s not going to be easy to change.  I don’t have the equipment yet I don’t really have a lot of storage and I’ll purchase when it is ready to be installed and it will too late then to change positioning.

You might consider mouthing the Era 300’s in the inverted position. Doing so may allow you to mount as high as possible and still have decent surround. TurePlay with and iOS device is now possible to properly tune to your environment. Here’s a link to mounting the Era 300’s in the inverted position:

https://www.sonos.com/en-us/guides/era300