I know this is probably locked down by apple, but I would prefer to be able to have the denon p4+ decoded audio-files downloaded for playback on storage, not just streamed, and thus also backed up to my enginedj install on PC. Anyone ever accomplished that?
Doesn’t that violate the terms of service?
If Apple haven’t agreed to offline storage then files cannot be stored offline. This applies to all DJ products with Apple Music. No storage, no stems.
Denon did once say, back in 2020, that offline storage would be coming - but then they quietly dropped the idea and removed all evidence from the internet.
Based on that, I’d say it’s unlikely to be resurrected.
Pretty sad. What use is the apple-music access on the denon then, if I can’t even use the music with engineDJ outside of the controller itself? I will remove it, and tell my girlfriend she can eliminate that subscription. I don’t use any apple/IOS devices anyway. Their loss.. 
So that you can use it on the Denon. If you haven’t got any Apple /iOS products then why did you choose that service?
Other services offer more functionality, but they’re still just rental services. The music doesn’t belong to you. You don’t get to keep it and do what you like with it.