TOTALLY anecdotal, and opinion only, but I tried both the Squeezebox Duet & Apple TV, though I didn't use either analog out--I fed digital coax (Duet) & optical TOSLINK (ATV) to a Cullen Circuits Stage 3 modded PS Audio DL III DAC. When I could actually get the Duet to maintain a wireless connection to my WiFi network, the sound was very good (wireless is a must for me--I'm not gonna run C at-5/6 cable, screw that). The ATV is the 160GB version. I synched ~130GB of my most loved music to the ATV HD, and I streamed the rest. Anyways, I ended up returning the Duet. Even w/out taking the Duet's/SlimServer's network issues, I would've gone w/ the ATV (and yes, ,my home WiFi network is rock-solid, thank you very much). I actually prefer the sound (again, optical to DAC) of the ATV. I'm not claiming any kind of big difference...it was VERY subtle; I suck at articulating sound, but everything just seemed to have more "weight" to it w/ the ATV--more bass slam, better dynamics. I had also assumed the Duet would win there. Of course, YMMV...
Maybe the fact that all my music is in ALAC, and that Slim doesn't natively support ALAC (transcodes on the fly) had something to do w/ sound quality? I dunno; don't care tbh. All I know is that, to me, the ATV output sounded just a tad "better."