Using the camera adaptor on the iPhone and iPad disables the volume control on the device - The only other way that may work is to divert the usb output to another output say the TRS outputs. The only issue i can still see with that is i can independently change the volume of each channel unless i use the expression pedal to vary the volume.
I have to inputs on the Headrush speaker but that is over the other side of the room and dont really want to keep getting up and down once i am settled