Good info ~ Thanks so much for the reply.
Is this behavior standard for CCK?
Is the iPhone just controlling the volume of the amp in the Microstreamer or is there actual attenuating going on on the level of the line out?
My apologies as I don't have anything compatible to test with here so I appreciate the info greatly.
I think the gain change on the headphone out is digital only, so lower gain than max means fewer bits are being used. It is a 24-bit device, so it is not a problem if it works this way; the limit in the dynamic range is the amp section for most portable DAC/amps.
You could ask in the microStreamer thread to be sure as the designer often answers directly about such things.