I stopped using airport express on my BS9000 via Aux because of the poor sound quality.
Yes it worked, it's nice to have an Idevice in hand and play sound via your B & O speakers but the sound is not good.
I'm talking about Spotify or Itunes here not better quality music if that exists.
And you have to set the volume way up compared to a cd listening.
A DAC or Playmaker must be considered. Just my opinion based on my experience (BS9000 / beolab 8000 and airport express).