Are you using the AMPPro 4? I have that in my 2020 Gt350 with B&O and the pink noise i played via CD into my DSP does not look like anything weird is going on with the signal as the volume increases.
I was worried about having to figure the signal part out but I don't think we need to be...
5 star car stereo in Clearwater has done 2 S550 Mustangs, the videos are great. I plan on trying to follow them when I get to upgrading my fronts(just finished replacing the stock B&O sub with a plywood enclosure).
Check out the following series from 5 star: