This is still unresolved.

When I called Sonos Support 3 weeks ago to report the Amazon Music Stations Dolby Atmos low volume issue, I asked about this Ultra HD issue and it was still being worked on. 

Hi @Jimmy C 

Thanks for your post!

Only a Generation 3 Sub can play DRM-protected music while not bonded to a Home Theatre device.

Therefore, first and second generation Subs cannot play DRM/HD content from Amazon Ultra while part of a stereo pair, regardless of what kind of speakers make up that pair.

For first or second generation Subs to play DRM/HD content, they must be bonded to an Arc, Arc SL, Beam (Gen 1), Beam (Gen 2) or an Amp.

Unfortunately, this is not going to change.

I hope this helps.

Ahh. This directly contradicts what I was told over the phone by a Sonos help representative some time ago (that for me, it was only a display issue) but it now makes perfect sense as to what is actually happening with my equipment and why I am unable to get Ultra HD with my bonded Sub (Gen 2) and two Play:5s (Gen 2).  Very much appreciated!