Applies to: Focusrite iOS Control, Scarlett 6i6 + 18i8 + 18i20 (2nd Generation), Clarett Range, Red Range
The headphone outputs on the 2nd Generation Scarlett 6i6, 18i8, 18i20 and the Clarett range are controlled using an analogue potentiometer, as well as the corresponding volume fader for those outputs in Focusrite Control.
In order to use Focusrite Control for iOS to alter the volume to your headphones, it's important to also have the headphone output dial on the interface itself turned up. You can then make adjustments to the level using the fader in the iOS app.
Conversely, the headphone outputs on the Red Range are digitally controlled and so can be adjusted exclusively from the iOS app if required.