I just went through my current Spotify playlist and everything was on there. I'll try it over the next few days. This may be it for Spotify.
Most of what I listen to on Spotify is not available. It is not that obscure.
Mike Pinto, Greensky Bluegrass, The Devil Makes Three, JJ Grey & Mofro, Jack Johnson, Dirty Heads... list goes on.
I guess I just listen to very mainstream stuff... the only in that list I have heard of is Jack Johnson. I guess it's fortunate for me that I have pretty vanilla tastes that Amazon seems to serve up.
Very fortunate from what i can tell. Looking at the top 100 albums in their "MP3 Downloads" section.. not a single one is available to stream via Prime
Sigur ros wasn't available.
Most of Animal Collective's albums aren't available as well.