Yeah, the Airport Express analog out is marginal in terms of quality. You want to use the digital output. Therefore, you need a mini-Toslink to Toslink cable. The other issue is that some apps (even under Mac OS 1.9.x, Mavericks) will not stream digital output without a helper app. I use Airfoil from Rogue Amoeba on my MacBook Air and my HP Z600 desktop. Unsure if it's bit-perfect, but it sounds pretty good...
I do Airport Express optical to my 12-year-old-and-counting GW Labs (they're gone - check Google for "Monarchy Audio DIP Combo") DSP digital reclocker, 44.1k no up-sampling. Out of the DSP to the Musical Fidelity M1DAC via AES Balanced. Analogue out to my Jolida JD3000B, fitted with NOS tubage. Is it perfect? No.
Does it sound great? Hell, yes.
-ds