Ah that's true. There is a pre-release app on the store atm called SonarWorks that seems to have Spotify Premium integration, granted they likely have some special channels that allow them to do that, maybe it could be something you might be able to look into. The app EQ doesn't offer much customisability and relies on their own database of 'referenced' frequency responses of headphones but obviously this limits subjective tastes and source matching.
I think making that available would be great help for many audiophiles who don't have to time or resources to maintain a wide library of files (and want to support the artists in some way and not pirate everything) but also want more control of their experience
