I have a poor sound quality on my Ubuntu 16.04 LTS (Dell Optiplex 790) machine.
All Settings / Sound / Test Speakers: works OK

Unfortunately when testing on youtube (here):
- it sounds MONO whole time
- "center" mode gives no bass line
- while headphones jack plug half way out: bass line on "center" mode appears, but "right" mode completely disappears

I've tried some solutions already - like changing alsamixer settings or amarok rhythmbox install, but so far nothing has helped.