Look at the green trace on the left. Find the number 100 at the bottom of the chart. Now look at the large dip that goes down to -15. That's the dip in response bman is talking about. That's not because there's a problem with his M5. It's because he has a jog in his wall at a quarter wavelength of 70Hz away from his M5. It's causing destructive interference as a result, right at 70Hz, causing him to lose 15dB of signal at that frequency. 15dB is 32 times less power than he should have at that frequency.
The red chart on the left shows the correction his software on his Denon made. It's better but still down 9dB or about 16 times less power than he should have at that frequency.
Crossing over at 80Hz, as he said, allows his dual subs to take over which I imagine gets rid of that valley.
A-LFR/1000-8 & 2/500v4x2
Air Force, Freedom