Something to add to the B&W discussion:
BMW is upgrading the B&W option next July ...
Per Ynguldyn, on the upcoming new 7-series:
Even better version of B&W audio.
Perhaps this may also be contributing to the shortage ... BMW may be thinking it's worth a production pause / slow down and then simply moving to whatever new tech whether or not it debuts on the new 7 series. Just speculating of course ...