Welp, I'm going to throw out what I know may be a terrible idea. Sorry.
What about taking a cue from ground-plane measurements? Often used for subwoofers. The idea is to put the subwoofer up against the floor so there is no reflection point. The microphone is also nearly at floor level.
What I am thinking you might want to do instead is to put your planars up against a wall and attempt the same thing?
Sorry I'm not familiar enough with DEQX to help more.