I am using my iphone5 and my ipad, together with a lighting to usb converter, to send music to various USB DACS. IOS7 will now permit exporting the digital stream. One limitation is that the idevice cannot provide power to the DAC, so if you get a USB powered DAC, you would need to send the USB signal through a powered USB hub. This should not be a problem with an externally powered DAC. There is plenty of conversation about this on computeraudiophile.com.
You still want to get an asynchronous DAC.
You still want to get an asynchronous DAC.