I have spent some hours looking for the answer but I just found conjectures (guess/believe/suppose) but no facts anywhere. Maybe Microsoft tell it somewhere but they manage to hide it for Groove Music passport customers pretty well. Is it because the bitrate is so low? In my phone, Lumia 925 with Windows 10, the sound could be described as slim. Although I'm happy that the low quality of the files are not hided behind excessive use of bass, as mp3 files of Spotify does, I still want to hear some bass.