I am really impressed with the sound quality of the Airport Express. I bought it used on Amazon and it was the first gen of the AX. I found this on Apples forum.
For analog audio, the AirPort Express (AX) uses a 16-bit Texas Instruments PCM2705 DAC. The AX only functions at a 44.1kHz sample rate. When you play 48 or 92 kHz data, iTunes sample rate converts the data in real time (to 44.1 kHz) prior to sending it to the AX.
All I know I am very happy to be able to access the same music I have in my main 2 channel system in my office now.
The sound quality is very good. Pretty easy set up once I reset it back to factory default.
I needed an analog connection since my mid 70s Pioneer receiver has no digital inputs.
My Wright WLA12 preamp and anthem amp 1 will be replacing the Pioneer, will still need analog inputs.