iPhone Best volume amplifying tweak for lightning (digital) out?

[thadoggfather]

I pipe my iphone in over lightning to 30 pin, no aux. headphone volume isn't problematic but with certain tracks, not being able to control volume when digital out and cranking my radio up it's too quiet whereas with the 6cd it can be deafening.

Anyways what would you recommend?

 

[Intell]

Audio out though the dock is line out and cannot be changed in terms of volume.