That’s a tough one. I love open plan homes, and I can’t think of a good way to occasionally close off a room - at least in a practical or attractive way.
Since you have planars, which are usually a little light in the bass anyway, perhaps adding a subwoofer would solve the problem and also be less complicated and more aesthetically pleasing.
Since you have planars, which are usually a little light in the bass anyway, perhaps adding a subwoofer would solve the problem and also be less complicated and more aesthetically pleasing.