Sent from my XT1060 using XDA Free mobile app
did u see a "volume limiter" message pop up when u try increasing volume while plugged in?
another thing that affects the sound is default settings for EQ (Wired stereo tab),
make sure it's at Effects: Custom, Surround: Off, Eq: Off (or whatever u like).
Compared to my older phones, its volume output is normal.