One of my biggest problems with the 20S is setting the volume for my GPS, smart phone, and intercom. If I crank up the volume for the phone (Galaxy S5), the sound from my Garmin 660 is distorted to the point of being almost unintelligible. If I drop the volume for the GPS, the phone can barely be heard. And the intercom volume suffers, too. I have the phone's volume pushed up as high as it will go (past the "any louder and you'll make yourself deaf" warning level). There's no volume control with a Bluetooth connection to the 660 (stupid choice on Garmin's side). The only fix is to be able to set the volume for each sound source.
20S - individual levels for each source.