As a professional audio engineer,  I'm very used to exactly how loud my spotify is when streaming over bluetooth at a given volume on my car stereo.  Which is exactly why last week I was surpised to press play upon getting into the car and have my audio be much quieter than usual!  At first I thought it may be the usual culprits, either the volume on my phone isn't maxed or volume matching is on or the EQ is enabled or SOMETHING, but to my dismay I couldn't find any obvious solutions.  Then I hop over the my Apple Music app and load the same exact song, and lo and behold, the volume I'm used to!  For the record I'm using the latest version, 3.4.0 on the newst iOS on a 6+.  I only bring this up because to untrained ears , something being quieter is just going to be immediately percieved as "worse" and I know there's going to be a load of people comparing spotify to apple music on their iDevices in the coming three months and if they get tricked into thinking it "sounds better" because it's a few dB louder that'd be pretty sad.  Another point to mention is that this affects the headphone out as well (not just Bluetooth), even when set to identical volumes, Apple Music is significantly louder than Spotify.  Hopefully this is just a bug on your side and not some shady ploy by Apple.  If I can offer any more details that help figure this out let me know.