Actually, I beg to differ. As long as your post-Audyssey calibration level is above -12 dB, you're in good shape. My system sub level is set to -9 so I have a bit of play available if I ever encounter content I think is too hot.
The Denon and Marantz AVRs have limited internal headroom, and by setting the sub level too high, the bass may be clipped on heavy scenes played at or near reference level. The clip point depends on the sub trim, the master volume, and the source level. With the source level set at 0 (as is usually the case) and the master volume at "0", there is just enough headroom for a full-scale digital bass signal in all channels to pass without clipping. The higher the sub trim, the less signal that passes without clipping. This also assumes Audyssey isn't being used as Audyssey reduces available headroom as well.
For what it's worth, other AVRs and processors likely have similar limitations. The worst example I know of is Oppo whose Blu-ray players clip for *any* sub out signal > 115 dB if bass management and analog outputs are used instead of HDMI. Unfortunately, there is no workaround for the Oppos. This problem has been demonstrated with real world content such as the wormhole scene in "Interstellar".