I have almost similar problem on my car Head Unit. It's an ac8227l based model with 1GB Ram and android version 8.1.0.
Everything worked fine, Poweramp player was fine. I updated the firmware which made minor changes to UI. I had to reinstall the player and now volume is very, all other apps have better volume at 15 point( 40 max). Poweramp achieve same volume at about 27. If I disable DVC, the volume becomes same as of 15 points with other apps but to my eyes I don't think the sound quality is same, I maybe wrong.
Before update, I just adjusted the EQ from app, no other changes. After update just reinstall with same EQ settings.
Can you suggest what I should be checking?