I use iPhone 6s (128GB), quite happy with line out sound quality; IEMs TDK BA200, Etymotic ER4SR, Headphone Beyerdynamic T51p. The 6s can drive those gear well, set vol 60-80%.
When I use high impendance headphone like Sennheiser HD600,I add Centrance M8 as external DAC+Amp.(through lightning...