I got an Apple TV 3rd Gen a year or so ago to use with MusicMatch and my HiFi. I was really disappointed to discover that it really didn't sound that great even with a HDMI connection. I recently read that for some reason it resamples music from 44.1KHZ to 48KHZ. This is the equivalent to transcoding on the fly which is inevitably going to sound terrible.
The thing is, I really love the new Apple music service and I am tempted to buy the Apple TV 4th Gen in order to use the service with my living room system. But once bitten, twice shy. I haven't been able to find out if it handles music in the same way. I haven't even been able to find any serious reviews of its audio quality. John
