Bluetooth by nature, will always have inferior sound quality compared to a direct wired connection.
Bluetooth (4.2) can only transfer at a maximum of 25Mbps - that's around 3.25 megabytes per second.
The Lightning port in current iPhones meanwhile works at USB 2.0 speeds - that's around 480Mbps - or around 60 megabytes per second.
Edit: missed the previous quote vs reg Bluetooth and cannot delete the post, sry.