Assuming AVC is either off or working, does the volume stop increasing at 3/4 scale for anyone else? Easy enough to check, try it sitting still so you can eliminate the entire AVC thing and any affect from engine, road, and wind noise. If it's like previous Harley audio systems, the quality of the sound as you near full output will likely be pretty poor and full of distortion. I was under the impression Harley had reconfigured the amps after having so many speakers and amps fail on the Street Glides not long ago. The easy fix for blowing speakers for instance is to limit amp output, versus cleaning up that output to eliminate the speaker damaging distortion.
Just thinking out loud.
Jerry