And as if by magic in I pop
MULF2 ($633, $6FL) = Bluetooth calls + AUX + USB, with optional Album Art.
Combox ($644, $6VC, $6FL) = Bluetooth calls and music + AUX + USB, with optional Album Art.
Yes, you have the MULF2 module, so you can't add music streaming to it without surgery (ie; a Combox retrofit).
Certainly try cleaning your AUX socket with proper switch cleaner. The socket is susceptible to Verdigris, which will affect the connection. Also depends how good the mmi (CarPlay) box is, and how the settings within the box are set up, as some can have a graphic equaliser in them. Also you may have a volume setting in the box, I'd not set it to max as this may distort, and you also have an AUX input level within the "External Devices" menu in the iDrive (scroll over AUX, press the controller and then turn the controller left/right to again the higher level. This one can be max'd)