It is possible that the low volume issue is a result of the max-volume cap, please follow these steps to try and remove the cap:
Remove the max-volume cap
(1) Open Settings
(2) Scroll down and tap on Music
(3) Check the label “Volume Limit” .. it should be OFF. If not, tap on it it.
(4) Swipe the slider to the right so that the volume is full. When you set the slider completely to the right, it means there’s no volume limit set.
(5) Go back (tap on the < arrow from top) and check if the label for Volume Limit is OFF.
(6) Quit and re-open the music app to see if the volume problem is gone.