Hi @Remy de Fromage 

Thanks for your post!

Yes, probably.

Please be aware, however, that TruePlay Tuning does not yet support inverted Era 300s so we don’t currently recommend it, but several users have already reported using Era 300s in an inverted orientation without issue.

I hope this helps.

Thank you. Because TruePlay Tuning is not supported, I’ll wait to raise the speakers until the day of TruePlay support arrives. 

I have the same ‘issue’.

My accoustical ceiling doesn’t bounce back the height channels of the Era 300 (and the Arc of course).

what could be a good solution for this?

Is there perhaps some kind of ceiling mount available?

Solution for Era 300 has already been provided. 

As far as the Arc, I’m not sure there is a good solution.  Is it possible to put replace the ceiling or put some sort of reflective panels or coating on the ceiling?  I doubt that can be done in a cost effective or aesthetically pleasing way.  Depending on size of the room, a Beam Gen 2 may be a better option since it does not have upfiring height channels, it simulates the height channels.   

Or you could just go without atmos content either by changing settings on TV or source to block atmos content if possible, or switching to a optical connection rather than HDMI-ARC, since optical cannot carry an atmos signal.  And obviously, you can’t utilize an Era 300 well in this configuration.

If you really want atmos, then perhaps a different system that uses ceiling speakers for atmos is the better solution for you.