Spotify's encoding (vorbis 320kbps) should be transparent at the maximum bitrate. However, it's possible that the files uploaded on there are mastered differently, for average consumer consumption instead of the full dynamics of most source material. I know SoundCloud enforces "loudness" mastering with presets when uploading for example.
The real reason Spotify's quality is inferior to others is that, if you have the music files, you can apply in-app parametric equalization on every platform and compensate for imperfections of your output device.