40-80 Hz dip? The middle frequency of that range is 60 Hz. The length of a 60 Hz wave is 18.8 feet.
Your room length appears to be about the same based on your posts. My vote is that this is a modal issue.
I agree with earlier advice to move your subs and adjust the gain on your subs to keep room response and reinforcement where it needs to be (as close to reference as possible). If your front and rear speakers are capable of good spl anywhere in the 40-80 Hz range (and I'd be surprised if they weren't) adjust their toe and positioning too. (Especially the rears where it appears you have room to play with.)
After that, employ bass traps in the back corners. Curtains, book cases, etc. won't attenuate bass.
