I've had three Xperia Z3's this year (which I've loved). The first blew out the headphone socket, so minor movements paused music (grr). The second was skewered on a surprisingly angular bollard in Toulouse airport, creating a pocketful of glass shards and a phone comprised of 60% duct tape.
Third time's (hopefully) the charm, so I've installed an SD card and (finally) synced my Spotify account. Unfortunately:
Android 4.4.x + Spotify + SD card != AMP (Amazing Music Phone)
Apparently, the issue is the term order. Switch 'SD card' and 'Spotify' around and music syncs to SD automatically. Mysterious and frustrating, but thankfully fixable.