I just received an iPhone 6s and have restored from my 6 to get all working.
However, the music was not set for offline play, so I clicked to make various playlists work offline.
In doing so I got a message indicating (I clicked through it before noting all the text) that I should go to iTunes on my computer and remove some devices for this to work.
I can see the authorise and reauthorise computers - but that is not phones or tablets.
The Apple Music help says up to 10 devices can be enabled, but nothing to explain how you would remove a device.
Any advice/comments?