The couch should be placed such that your seated ears are not at 1/2 the room depth. Either move forward or backward and test all the sub positions again. Also, given that the room is open to a much larger area, your sub is trying to fill that entire area, and failing. So, yes, closing off the room would probably improve matters greatly. However, a curtain is not going to be effective in this way, you need real walls and a sealed door ... or multiple larger subs.
It's a VIRTUAL channel unless stated otherwise.