Airport Express is capable of outputting both analog and digital signals. Based on the foregoing, I would guess that if you are using recent Outlaw gear or equivalent you would be better off using the optical digital output of the airport and connecting that into an optical digital input on your receiver or pre-amp/processor. Make sense?

What to you mean by "stereo input"? I take that to mean analog input (but the digital is also stereo so it's better to say analog or digital).