Similar issue – volume is too loud when using iTunes/Airplay as compared to MPD.

I'm seeing that RaspyFi configures my DAC, an HRT MSII, without a softvol Master Volume control in alsa. That means the samples are sent to the DAC at 0 dB gain (??) That's ok as long as player application has very fine grained volume control. MPD seems to have fine grained volume control but iTunes does not, particularly when approaching zero-volume (silence).

iTunes volume stepping is quite coarse. More precise are the OSX and Win7 System (Master) volume controls but even those start flaking out as silence is approached. Windows is a bit better than OSX in my testing.

Also there appears to be a bug in iTunes 11 Airplay where it fail to set OSX (Mavricks) audio output device. This will screw up volume.

Another issue is whether or not a System (Master) volume control is even enabled by the OS for a given audio device including Airplay. Here are some test results with HRT MS-II DAC that show how the audio subsystems in OSX Mavricks and Windows 7 handle Master Volume assignment and Device Listing.

DAC Connection—-Master Volume—-Listed as Audio Device
———————————————————————————
OSX USB—-No—-Yes
OSX AirFi—-Yes (1)—-Yes

WIN USB—-Yes—-Yes
WIN AirFi—-No—-No (2)

(1) This is a bug. The iTunes Airplay control does not automatically set Audio MIDI Setup to "use this device for audio output". This must be done manually before Master Volume control shows up.

(2) Since Airplay is not even listed as an audio device in the windows Sound control panel there will never be Master Volume control.