do you have an sound pressure meter (even a phone app one).
I'd measure at your typical volume settings.
Not sure how the volume scale on the Onkyo runs. Sounds like 0 to xx, where 0 is no volume (so possibly an absolute scale).
Anthem MRX has a relative scale. Runs from +10 to -90 I think. I'm typically around -30 or -35 for music and its reasonably loud. Running a 100WPC Atlas Magnum for my fronts. If you are planning to use onboard amps of the Anthem, the 1120 and 720 are same wpc (140 for channels 1-5, 60 - i think - for the additional) 1120 = 11 amplifier channels, 720 = 7. The 520 is 100 wpc., These are at 8 ohms and 2 channel driven. If you want/need/will want Dolby Atmos, need to go with either the 1120 or 720. If only 5.1 or less, 520 should be fine for your listening habits described above. Can always add an outboard amp.