If you spend the time with placement, you can drastically reduce both peaks and nulls. In the 40's is tough and is generally best addressed via placement prior to treatment. To get into the 40's, you'll need 6" minimum.
Also, you can treat the corners all you want but if it's a modal issue based on width or length, the corners won't address it. It's likely the rear wall and that's what would need to be addressed.
If you can get to a point where you can cut the null in half or move it higher in frequency based on distance, you'll have an easier time.