If you connected the line out from the Amp to a line in on the Denon, it could play anything the Amp could play…with one caveat. The analog line in on the Amp has a 75ms delay, so anything you send from the Denon to the Sonos system would have that 75ms delay.
You’ve gone a step further, however, by adding an Apple TV. While if the Apple TV is playing via the digital input (HDMI ARC), it will play on that particular Amp, and any Amp ‘bonded’ as surround speakers at the same time, with lipsync. Any other devices with speakers (the third Amp, or your Denon device, will be delayed by 75ms, due to the nature of the Sonos.
The difference between a Sonos Port, and a Sonos Amp is essentially the Amp has a, well, amplifier in it that can drive its own set of connected speakers (and some slight input differences.
If your ultimate goal is to play speakers in other rooms, you’d likely want Amps to drive each ‘room’ (essentially a pair of speakers, although can go higher, depending on the speakers). As long as you’re not concerned about 100% sync, and can live with a 75ms delay between the TV room, and any ‘grouped’ room, than the ARC output from the TV set can be sent to a Sonos Amp. If instead, you’re more interested in playing music in sync, then a Sonos Port connected to the Denon will work, although any and all Sonos speakers will be slightly related from the Denon speakers (but all Sonos devices will be in sync).
I feel like I’m talking around all sorts of potentials. What is your ultimate goal?
