I use the BB music gateway in my car and it works great - level isn't an issue, but there is a setting to adjust the input level in idrive, have you tried that or do you not have idrive/nav also? Not sure if there is a way to adjust Aux input without idrive, in the older cars, the button with a music note on it would let you adjust the input level when on AUX, but seems that has changed on newer vehicles.
2008 335i Vert Montego/Saddle/Grey Poplar
2011 Audi S4 Deep See Blue/Black/Stainless
2014 X3 35i M-Sport Space Grey/Chestnut/Fineline Wave