Before running the ARC measurement it is important to adjust the gain on the subwoofers. If you only have a single subwoofer, this can be done by running the “Launch Quick Measure” tool, and adjusting the subwoofer gain dial/knob until the on-screen measurement reads close to or exactly 75 dB.
If using multiple subwoofers on an MRX/AVM model without independent subwoofer outputs for each sub in the system, they should be balanced to one another before calibrating the rest of your system. Set each subwoofer's level dial/knob so on-screen measurement reads 71 dB for each sub if using two subs, 67 dB for each if using three subs, and 63 dB for each if using four subs.