When connecting in the car make sure the "media" volume on your phone is turned up to at least 90%. This will eliminate the need to crank up the volume on the car stereo. I have mine connected to a Tunelink BT connection in my Equinox and the MyLink system in my 2013 Camaro. It works great!
There is an app called CarHome Ultra. It is basically a car dock app, which I use a lot. I am on the road for a living as a territory manager for a financial institution so having my car wired right is critical. CarHome Ultra has a setting that allows you to set "media" volume to a specific level when you launch the app. You can add specific applications to CHU and run them from within the app. I have Nav, Maps, PowerAmp, Pandora, XM app, several contacts, etc, along with pause/play, skip and back for music.
You could also use a TecTile to set volume when getting in the car as another option.
On another note: if you have the equalizer set in your music app and the tones or equalizer set on your car stereo it can over compensate and sound like crap. I am using PowerAmp as my music player and it is a one touch process to turn on or off the EQ.