Have you tried moving your sub around and playing with the BFD now that you have it Fred? Maybe you can find a more suitable position that you can tweak slightly with the BFD?
I don't have a lot of placement options, but the back corner placement did get rid of the 40-45Hz null, so I will give that a go.
My feeling is that what we call boom is created by reflections. What happens is that the reflection of the 'attack' part of the reflected signal tends to drown out the natural decay you are meant to hear. Once that note/signal bounces back and forth several times, it changes the character of the sound resulting in boom. The only way to deal with that is bass trapping, and for low notes that takes a LOT of traps
