I'm currently looking to buy the SR15. But I have one question:
Does it support the Amazon Music app? Online and offline? Because even if the quality isn't that great, I actually have quite a huge library there and a lot of free music.
Appears that Amazon Music is one of the supported apps.
Open APP Service supported APP
7digital Music / Amazon Music / Apple Music / Audible / Awa / Bandcamp / BBC iPlayer Radio
Bugs / Deezer / Dsaudio / FLO / foobar2000 / Genie / IDAGIO / Joox / Kkbox / Last.fm / Melon
Moov / My Yuner Radio / nugs.net / Onkyo HF Player / Pandora / Player FM / Qobuz / QQmusic
Sirius / Soundcloud / Spotify / Tidal / Tunein / xiami music / Yandex Music
There are 33 validated apps by Astell&Kern, Some APPs which requires certain steps are incompatible such as Google. Although the APP is validated, there might have some restrictions on services, policies or updates.
(Supported APPs will be continuously updated.)