I do not understand the Preferred Format option: it seems to be activated only if Bit Perfect is disabled. What is the interest ? Why would you change the sample rate at the level of the computer before sending the data to the D Premier ? Although how and what is done exactly is not explained at all... Why would you downsample the data ? or upsample them ? The only reason would be if the network does not support the high res files and data needs to be dowmsampled first.
There are technical reasons for this.
Keep in mind, that Devialet AIR starting with version 2.0 is now operating in a way that it makes your Devialet amplifier to a remote sound card of your computer. This is true for both Mac OS X and Windows.
But a sound card has to be able to play sounds from different sources simultainously. So you can play a sound from youtube and then a new mail comes in and makes a signal sound or you even can have a voice chat at the same time.
All of this sounds that have to be handled by the sound card come from different sources and could have different number of channels, different bit width (8 Bit, 16 Bit, 24 Bit) and different samle rates.
So the sound driver in the operating system has to convert them to a common bit depth and sampling rate and mix all that different sounds. This common bit depth and sampling rate ist the one that is set as your prefered sampling rate in the AIR settings.
This is nothing Devialet specific by the way, for every sound card in your computer (as long as it supports more than one sample rate) this setting has to be done. In Mac OS X for example this is done with the program "Audio and MIDI settings". And indeed changing the preferred format in the AIR settings does nothing other than changing this setting in the "Audio and MIDI settings".
So now what about bit perfect.
From computer point of view, this is the special thing, and here the differences between Windows and Mac OS begin.
Windows offers the WASAPI exclusive mode for programs that want exclusive control over the sound card. In this mode only one program at the same time can output sound and it will be played in the format the program requests. So in this mode bit perfect (no conversion of bit depth, sample rate and no mixing with other sounds) can be realized.
But not all sound-players on windows support this exclusive mode of WASAPI, some only support the shared mode of WASAPI and that mode works like described above. Unfortunatelly iTunes on Windows does not support the exclusive mode so that bit perfect output is not possible with iTunes unter Windows (if you have songs with different sampling rates for example in your playlist, it will not switch to the native fomat of the song but always using the fixed "preferred format").
On Mac OS there is even no such equivalent like the WASAPI exclusive mode. There is no exclusive mode at all. Everything is always shared. So Devialet had to use a trick to make bit perfct possible on Mac OS. But this only works together with iTunes.
AIR 2.0 is not only a sound driver for Mac OS, it also monitors which song iTunes on Mac is currently playing and requests the operating system (Audio and MIDI settings) to switch the sound card to the format used by this song. So everytime a song has a different sample rate or bit depth than the currently selected one it will modify the "sound card settings".
But there is nothing to block other sounds out on Mac OS. The only thing that will not be routed to the devialet are system sounds. So to get bit perfect on Mac you have to use iTunes (in contrast to Windows where you should not use it), have to activate this bit perfect setting in AIR to enable sample rate switching have to keep the volume slider in iTunes always at maximum (very important) and disable all EQs and volume normalisation settings in iTunes (but they are by default off).
But you can use the volume control of OS X because this will effect the volume control in the Devialet. Remember: The volume slider in iTunes affects how loud the sound from iTunes should be mixed to the system sound, so it affects the mixing which destroys bit pefect. So keep this always at 100% and change the volume with the Devialet remote or the system volume of Mac OS X (for example with the keys on the keyboard).