Ok, maybe this is interesting. Instead of moving the speaker, I moved the mic approximately 1ft backwards (away from the speakers and the front wall), forwards (all the way up to the edge of the desk), left (same distance from front wall but directly left, so it's closer to the speaker), and right (same distance from front wall but right, farther from the speaker) from the mix position. As you will recall, the room gets narrow as you go left because the back wall is diagonal. Here are the waterfalls. As you can see, aside from the single null, the mix position looks really good. Moving the mic forward simply amplifies everything, not really fixing the null. Moving the mic back just 1 foot makes everything fall apart with 2 new nulls appearing higher up. The 43Hz null is still prominent, but not as much as before. Left looks pretty good across the board and the null is better than any other measurement. The null returns when I move the mic right of the mix position. Thoughts??