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I've just set up a new chromecast audio + mini amp + bookshelf speakers. I listen to all my music through spotify premium and I'm please with how it works - open spotify on my iphone, press play, select chromecast directly from spotify app, music plays. It sometimes takes a few seconds to connect but nothing that's bothered me too much so far. This is all on a non-bt router if thats any help.
I've got another couple CCAs on order to attach to existing speakers as you can select them individually or set up different groups (i.e. whole house, all downstairs, kitchen+living room etc)
Denon i think has their own range or wireless speakers and system. I find it costly but you may be interest if you basically need everything. (I only suggest it as you mentioned Denon)
Im going through a battle now with it myself. If they dont mind annoying proprietary apps, then Chromecast or Jongo. I have the Jongo, but it annoys me as im forced to use their app to play music.