I have asked various versions of this question of iPhone as a direct digital source that bypasses the iPhone/iPad/iPod's DAC and uses an external DAC, in my case the DAC in my Preamp, and rarely get confirmation.
An Oppo I own is a great experience even allowing me to use the Oppo remote to control the iPod I have connected.
"The HA-1 passes Apple's "MFi" certification and is compatibile with the latest iPod, iPhone and iPad to be used as the digital audio output accessory of these devices. By tapping directly into the digital audio signal of these devices and converting the audio to analog using the high performance SABRE32 Reference DAC, music from your favorite portable device will sound its best."
As I researched a preamp for a new 2-channel room I had trouble getting a strait answer. As far as I can tell most preamps with a USB DAC allow connection from your iPhone/iPad/iPod via the Apple Camera Connection Kit. This should bypass the i device DAC and allow me to play my Apple Lossless files I've saved in iTunes using my preamps USB DAC, although not with the control of the HA-1. I wonder why so few manufacturers offer this. License fees? There are a lot of Apple owners out there, granted the HA-1 is a headphone amp with preamp capability.