Too bad the digital connection on the BS9000 is output only.
As Stan says, you have a couple of options.
The Airport Express (AE) is digitally transparent if you use its optical TOSLINK minijack output, unfortunately the BS9000 doesn't accept that signal. I would recommend buying a LAVRY DA10 stand-alone Digital-to-Analog converter.
http://www.lavryengineering.com/productspage_da_10.html
You can connect the Toslink optical cable to that, and then run a pair of RCA PHONO wires to the Extra Equipment connector on the BS9000, using a 5-pin DIN-to-RCA PHONO converter plug.
To get the best possible signal, you should try to get an integrated 5-pin DIN to RCA PLUGS cable, in order to eliminate a junction - the fewer the better. Like this one:
http://www.flashbacksales.co.uk/acatalog/5_Pin_DIN___Phono_Cables.html
With this setup I guarantee you'll hear a difference between AIFF and compressed files -- but most of all: you won't believe your ears.
With quality cables (don't go overboard here, just make sure they are of an acceptable standard) and the LAVRY DA10 this will run you to about € 1400.
I assure you it's worth it.