We haven't actually established it's a null yet. Download some test tones in the 50 Hz area you mentioned, copy it to a CD, not mp3. Play that and walk around the room. If the null is as big as you think, you should notice variations in volume, without a meter. If the volume is consistent throughout the room, you got other problems.
Acoustic treatment in that frequency range is not trivial. However, I once had a hallway closet beside the living room that sucked 90Hz if it was closed. Sounds weird, I know, but I had to open that closet every time.
Acoustic treatment in that frequency range is not trivial. However, I once had a hallway closet beside the living room that sucked 90Hz if it was closed. Sounds weird, I know, but I had to open that closet every time.