No idea about the analog out, but the digital is essentially perfect.
I used the original AE for many years and still dig it out on occasion. I also have the later generation. I’ve not seen measurements anywhere, but know that it is bitperfect because I was able to stream DTS 5.1 Music tracks via the optical out.
Another sadly abandoned attempt at surround music, DTS Music was a mildly compressed 5.1 surround format that can only be decoded if the stream is bitperfect.
The earlier Apple TVs with optical output are much easier to use than Airport Expresses, but convert everything to 48k.
Whether this is audible for most music is debatable, but for DTS it is. You just get hiss.
So if the job of a music streamer is to simply deliver bits to a DAC without changing them, the ancient AE does.
As far as I know, no other current Apple product (aside from the Mac Mini via USB) leaves standard digital untouched.