I've noticed the headphone level is a touch lower on my iPhone 5 than it is on the iPhone 4. At work, we have a bar area where people can plug in their iPods etc. to provide music and the level of the amp (set to max) is noticably lower when I plug in my iPhone 5.
With the iPhone 4, you had the option of using a dock to provide an analogue audio output at line level, which also be marginally better quality due to impedance matching with a line level input on an amplifier.
I realise there is unlikely to be an Apple Dock for the iPhone 5, but thinking about the Lightning to 30-Pin adapter that has a built-in DAC, would this be an option that provided a Line Level analogue audio output?