Car streaming blues, ain’t marketing great

Try downloaded tracks from a streaming service playing from your phone to either a digital USB input, which would be best, or simply an auxiliary input mini jack. If you have to use the jack, look for a dongle the DAC between your phone and your car. 

Both of these methods will be far superior to what you’ve tried.

I stream Spotify and Radio Paradise from my phone via Bluetooth in all 3 of our vehicles. None of the vehicles have top of the line factory sound systems, but they all sound fine to me. I sometimes use CarPlay in our 2019 Alfa and it sounds good and it's nice having the Spotify or Radio Paradise display on the in dash infotainment screen. 

Harmon Kardon has been making car stereo since at least the early 80's (I use to sell it). It's not top of the line, but usually quite decent. I agree with reubet and M669326 as your first options, if that doesn't work out, I'd suggest upgrading the speakers, Polk, Infinity, many decent brands out there for upgrading. Upgrading the electronics nowadays is either impossible or difficult and expensive for most cars.

I also have XM Sirius in my MB and it sounds fine, not HD, but it rocks fine for road trips. Hope this helps.