I'm kind of shocked how poorly the Denon is shaping the crossover on your mains. Looks nothing like thecorrect correct slope of an 80Hz crossover? Do you still have some Audyssey processing enabled? I ask because why would there be a +60dB level for 40Hz with a 80Hz crossover?
All that extra energy (and whatever Audyssey sauce has been added) can certainly impact the combined response with the subs.
JVC X750R, 133" DaLite HP (2.8 gain), OPPO UDP 203
Denon X4300H, DIYSoundGroup Fusion 15 L/C/R, Mirage FS V2 Surround, Mirage Omnisat V1 Surround/Atmos