A lot of variables come into play mostly room related. Kinda looks like it is bass management related but you said its at 80. Make sure any listening mode is disabled. Do you see the bounce if you measure just the fronts or just the subs? If you do it's a bounce or reflection. If you don't, you may be able to adjust the phase first on each sub to get them to their loudest/smoothest combined output with the mains. Then try adjusting the distance on each sub independantly to see if that helps the dip. Also if the tip of mic is pointed straight at the speakers make sure you use the 0 degree axis microphone calibration file otherwise most of the time you'd use the 90 axis for room measurements with it pointed straight towards the ceiling.