Using Spotify Connect on the Mu-So 2 is very convenient, but it suffers from the same thing as many other compatible Spotify Connect devices do. That is the sometimes enormous volume differences between tracks, forcing the user to juggle the remote constantly to adjust playback volume.
It seems that Spotify has now implemented the required API part for the eSDK. The SpFormat struct now includes gain_mdb that indicates how much gain to apply to the current track. API Reference eSDK 3.203.233 | Spotify for Developers