You don’t have a lot of choice, do you? Walk-mounted on the side walls so to minimise and sound deadening by the curtains. I’d put them a little bit lower than the Ones though. Leave at least 8” from the back wall. 

Since the Era300 only emits sound from the sides and top (when connected as surround) your only option is as in the picture.

However, I would place them at head/ear height when you sit down.
If you have the opportunity, I would pull the sofa out a little from the wall so that the speakers are behind you.

My Era300 is also placed far from optimal as my sofa is in a corner, the good thing about the wall brackets is that they can be angled in all directions so you have a little room to play with the placement/angle.

I have heard you can place them upside down and closer to the ceiling, Trueplay should be supported this way but I am somewhat skeptical of this placement and there is not really any data out about what the sound is like when they are placed upside down.

Thank you for the suggestions!