Um how does lossless work over an analog headphone jack, exactly?

No doubt there are “night and day” differences compared to plain old AAC256 streamed direct to the Sonos player...

Personally not noticed any difference between lossy AAC and ALAC via Sonos but there are subtle differences between AAC and lossless/Hi Res  which have made a massive differnce to my listening pleasure on my hi-fi. Does sound less compressed, more natural and can’t stop listening to it as I am hearing details on songs that I have know for a long time that I have never heard before. My hi-fi has never sounded so good since started playing the new Apple Music Lossless direct to my my hifi rather than through Sonos. Sounds amazing. For sure though on Sonos and bluetooth headphones then absolutely no difference at all.   

Sonos doesn’t yet support Apple Music ALAC streaming, only AAC.

For Bluetooth headphones you’d need LDAC or aptX-HD to do justice to lossless, hence not Apple. 

Yes completely agree with you. I wasn’t comparing on AirPlay as AirPlay doesn’t support lossless yet. I was comparing AAC file amd ALAC files direct on Sonos which to be honest I can’t hear the difference.   And yes Apple Bluetooth doesn’t support lossless but I would not expect to be able to hear the difference outside on a Bluetooth headphones/earphones anyway. The sound quality of Bluetooth headphones isn’t good enough. The big difference is between playing Apple Music lossless directly on hifi rather than through Sonos. 

According to Wikipedia Airplay is lossless. It also looks like a lossless bitrate as measured, at least when streaming FLAC through the Deezer native app to Sonos. Maybe it doesn’t bother with Apple Music? 

The sound of BT headphones is perfectly good enough. Get some Sonys with LDAC. 

Yes agreed. Understand from manufacturer tech teams that AirPlay is capable of lossless just not enabled on Apple Music yet. I have some Senn momentum 3(? BT headphones with apxhd (?)) which barely use and tried various Bluetooth earbuds - fine for out and about/sport but for me nothing compared to my wired headphones. Each to their own. 

Agreed that wired headphones are best (I also use Senny HD650 with Schiit Magni) but Sony over-ears on BT/LDAC are still very good.

BTW I understand Senny Momentum 3 do aptX and aptX-LL, not HD.

Good set up. You are rignt the Sen Momentums only have aptX. I like them but they don’t quite do it for me and don’t get much use since i got a wired Senn headphone/dac/amp set up. I tend to believe bits are just bits and am skeptical about hi-res and cables/connections. But my set up just sings and I can hear what the fuss is all about since i sourced Apple lossless/hi-res music from iphone via adapter and a USB cable for whatever reason. I’m surprised such a basic and relatively cheap connection set up could improve sound so much. I’m  curious whether a dedicated streamer could get any further improvement but will wait to see what hardware solutions are developed for Applle Music lossless given my guess that Apple woould introduce lossless was correct.