I just yesterday purchased Apple TV 4K 32GB model. I first plugged it to my computer monitor and did initial setup and then updated firmware to 11.2.6 version and loaded Netflix, HBO and so on. Then I moved it to living room and plugged it to my Yamaha RX-V1800 which it connected to my projector (BenQ W5000). Everything seems to work and automatic refresh rate for Netflix 24p content also appears to work OK.
However my first impressions was that sound quality, especially dynamic is decreased in Netflix. I had previously WDTV Live and it did output bitstream DD+ in Netflix but Apple TV wants to do decoding by itself and output PCM. I know that should not make any difference but I could swear that audio dynamic was completely lost. Everything sounded quite muted even I upped volume. All special effects sounded compressed. If I switched audio output mode from default best to Dolby Digital all audio dynamic was restored but audio will be down sampled from DD+ to DD so definitely not optimal solution. I had many years ago PS3 fat that I used a long time to play blu-ray and it did decode audio and output them as PCM and I had zero issue with sound dynamics. I now have PS3 slim and also couple standalone players and if I switch those between PCM and bitstream there is absolutely no problems, so I know my receiver can handle PCM just fine. Is there known issue with Apple TV 4K PCM or what might be the culprit for bad audio? I have made sure I have not dynamic range compression selected in Apple TV, but it definitely sounds like it is doing that and hard, I mean usually in my another devices trying that option has very small effect in dynamics, nothing like what I've been experiencing with Apple TV.
Also I got feeling that image quality in Netflix was not always as sharp as it was with WDTV Live. Especially I got that impression when I tried 1080/24p content which I selected in Apple TV settings to output as 1080 SDR 24p, so it should not do any conversions. However I felt that material that was encoded in 50p looked somewhat better. I live in Europe so Netflix has also some 50p content.
I noticed that there was no way of getting stream information from Apple TV as it is possible with WDTV Live so that I could verify I was really getting 1080 stream but I see no reason why it was not that because my network is capable of that and I never had any problems with any other devices. I also ran download speed test in Apple TV to verify network speed and it was exactly what it was supposed to be.
So in the end, I finally got device that automatically can adjust refresh rate in Netflix (in WDTV Live I had to do it manually every time I used it, and often go by trial and error to get right refresh rate selected for the movie I was watching) but I got two problems - lost audio dynamics and softer looking image quality.
I hope this is just TV OS problem but I must admit I haven't been doing my homework by reading about Apple TV user comments and reviews of audio and video quality, so maybe this is just what this box is able to produce. I'd be appreciated to hear more from others what they think. Another reason I got this Apple TV was that my received has no free HDMI inputs and I desperately needed to get HBO and few other local streaming services for my projector and I did not want to add any HDMI switch to add one more device so I replaced my trusty old WDTV (which is basically limited to Netflix app only) with it.