I have been trying the free trial of Amazon Music HD. I was all set to keep it up until I found out that I can not set it to use Prime Music on the Echos and the Music HD on my phone. (My primary device for listening to music.)
What happens is I will be at work listening to music. At home my wife asks Alexa to play some music. Instead of using Prime Music as it did before the Echo just hijacks the Music HD. My phone stops playing without warning. It gives no indication that is what has happened. I normally use the Google Play Music service and think I am going to end up going back to it despite preferring Music HD. I am not going to pay £7 a month more for the family HD plan when my wife listens to music for about 2 hours a week and dose not care about the quality.
Having looked at the settings it appears that Amazon group their music services (Prime Music, Amazon Music Unlimited and Amazon Music HD) all under the one umbrella/setting called Amazon Music, so there is no way to separate the Prime Music from Unlimited or HD
It is looking like Amazon Music is not going to be for me. My wife listens to about 2 hours of music a week. Which with Prime Music is included our prime subscription. I currently pay £9.99 for the Play Music, which I use on my phone. I would have gone to £12.99 for the better quality. But £19.99 (Or £16.98 if I go single device route.) It becomes two expensive.
Maybe worth a revisit in the future, as otherwise I was loving the service.