I think I might have fixed my Android Auto sound quality issues. Basically, the sound coming from any app (primarily Amazon Music but also Google Play Music) was awful. Harsh highs, muddled mids, and barely any lows. It would actually cause me physical discomfort to listen to. Equalizer settings on the head unit couldn't do much to compensate since it was too out-of-wack to start with, and also it just sounded super compressed.
I discovered it sounded great on Bluetooth, so that narrowed the problem down to something phone-side. I did some Googling and found the recommendation to "Disable USB audio routing" in the phone developer options. I tried that, and boom, huge improvement! Much richer and clearer sound. I was switching between AA and BT in the middle of songs to compare sound quality, and as far as I could tell it was pretty much identical. People online have guessed that setting makes AA have to get the audio from BT instead of through the cable, I might do some testing to see if that's actually what's going on.
I do still have the issue where AA reverts to the startup volume between every track, but that's a lot easier to live with if the music actually sounds good. From what I've read I think that's a firmware bug.
Part: Noname 187B
Firmware: 5516
Phone: LG G6, Android 8.0
Gotta make sure it's good to go for the new Tool album