Apple just released detailed specs on how to create the best sounding master for iTunes publishing. When can we expect similar guidelines from Spotify?
They are also addressing the loudness war issue, stating that the "sound check" option in iTunes (similar to the "set same volume" option in Spotify) is an important step to help producers and mastering engineers put an end to the loudness maximation, something most engineers want to do but cannot in the current state of distribution.
The "sound check" feature does not tamper with the audio, it simply lowers the volume on tracks that are mastered with artificial volume boosting. Can we say the same about Spotify's "set same volume" option?
Spotify was early with normalizing loudness in the desktop client which was very positive. To help things even further you should make sure that it is preselected in all new installations and also bring back the feature to the mobile client (why was it removed?). In an ideal world the option is not even selectable but always on, completely removing the need to artificially boost volume in the mastering process.
This may seem like a small technical detail but it is a huge deal for music producers and mastering engineers all around the globe, and in the end it would prove to increase audio quality for all music lovers.