[Viffermike]

One thing everyone is forgetting about in this thread is this:

There is no 'gain' or 'volume' to adjust before the HU/iDrive because the music is being transmitted in digital form.

Neither Bluetooth nor USB has the capability to transmit an analog signal. Therefore, both digital-to-analog conversion and gain application (basically, volume) to the analog version occur within iDrive.

The gain level is likely lower than everything else since BMW doesn't have control over what is fed to the car via Carplay, Bluetooth, USB, etc. To make the programming still sound OK even if it's of low quality, it applies relatively little gain to it. This is strictly conjecture, of course, but it makes sense considering the many other "tricks" BMW uses to make the 2 Series audio sound acceptable to most.