Pack your bags. Just Google it, or you can start here [1]. A relevant quote “This is because Spotify applies Loudness Normalization to your tracks as they're played to listeners”
A brief search shows that both Apple and Spotify offer 256kbps AAC. ABX testing these two would be pointless. They’re both above transparent. It’s NOT the codec.
Send me to heaven if X would allow for parametric EQ settings based on headphone/sound system. This would appeal to *actual audiophiles [2]. This would confuse the hell out of normals, so bullet point marketing is what we get.
[1] https://artists.spotify.com/faq/mastering-and-loudness [2] https://github.com/jaakkopasanen/AutoEq