Richard: Is one better than the other when listening to the same digital music track?
Interesting question !
I am surprised no one else answered.
There has been some previous discussion about the virtues of using a (e.g. Cambridge) DAC to provide a better quality input than straight from the Airport Express.
Also there is a question of whether you would lose quality by using Wireless (certainly mine drops out every so often) ... I have always wanted to see if the Airport Express would output if it was ethernet attached (thus getting around the drop outs) but I have never go to try it.
My "gut" tells me that the SD card sounds better - but I have not blind tests of the same track ...
My "brain" also tells me it should be better - less translations and components along the way ...
I hope my reply will stimulate more views/opinions for you
Update:
I just conducted a small test - All Angels singing Songbird (happened to be the first track on the SD Card) versus Airport Express. My son (better ears than mine) and myself could hear differences, but swapping back and forth we did not come to a firm conclusion as to which was better
I still say my logic must "prove" that the Airport Express link would have to be inferior:
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 ->
than the:
SD card -> BeoSound 4 digital MP3 input & decode processing to ML output processing ->
(I think that (maybe inaccurately) explains what is happening)
First B&O (1976) was a Beogram 1500 ... latest (2011) change has been to couple the BL11 with the BL6Ks *sounds superb*