High noise floor using compact Power Series amplifiers with PAC AP4-FD21 interface

How do I eliminate floor noise when using the PAC AP4-FD21 factory interface with compact Rockford Fosgate amplifiers?

When using the PAC AP4-FD21 (AmpPRO 4) amplifier integration interface for select ford vehicles, we recommend the below configurations:

PAC Interface:
AP4-FD21 = Set to 4V output (default is 5V)
https://catalog.pac-audio.com/catalog/amppro-amplifier-interfaces/ap4-fd21

Rockford Fosgate Amplifier:

Adjust gains per CLEAN setup for typical 0dB, +5dB gain overlap

Models Affected:
T400X2ad or TM400X2ad
T400X4ad or TM400X4ad
T500X1br or TM500X1br
T750X1bd or TM750X1bd
T1000X5ad or TM1000X5ad

This above configuration can also interface with the PAC APH-GM02 (speaker connection harness) for select 2014-2019 General Motors vehicle swith factory amplified sound system.
https://catalog.pac-audio.com/catalog/audio-interfacing/aph-gm02