If you care about sound quality, I would strongly recommend an external digital to analog converter (DAC).
Digital music must be converted to analog signals to enter the AUX port. The built-in DAC in PCs are notoriously bad, as is the DAC in Apple Airport Express. Fortunately the Airport Express has digital output as well so you can bypass its DAC. The following path gives you CD quality, provided the music isn't too compressed:
Wirelessly: iTunes on PC -> Airport Express
Digital cable: Airport Express -> DAC
Analog cable: DAC -> BS 2500 AUX port
The internal DAC in BS 2500 isn't bad at all, but you will easily get a better one as external DACs tend to be good. In that case you may actually improve the sound quality playing it uncompressed on the PC with iTunes, compared to playing it directly from CD on the BS 2500.
Sigmund