MacBook (iTunes MP3 decode processing to digital sounds packets) -> Ethernet cable -> Wireless Router -> wireless -> Airport Express digital sound packets to analogue output -> Aux Cable -> BeoSound 4 analogue input to ML output processing ->
Remember that the Airport Express enables you to output your audio through a digital cable. No reason to convert to analogue. You can even stream lossless digital audio to the Airport Express, via a digtial cable, to your device (as long as it has a digital input, like the BS3).
It's going to be highly negligible if you stream a high-quality audio track via the Airport Express, compared to the the SD card. ie. you would have to be incredibly picky and have super-sonic ears to hear any difference. Pretty much a moot point to me.