Lucid, I have the same exact issue running the Fusion RA 770 with JL Audio M800v2 with M600/1 mono sub amp. Using downloaded test tones from SMD on a USB drive. HU was set up to recommended settings per SMD. The DD1 would detect a signal and show the correct test tone but I could turn up the volume to Max (24) with no clipping detection.
Tested the DD1 on a different HU (Kenwood) and it performed perfect. Conclusion: The Fusion HU is free of clipping at max volume.