Spotify doesn't "master", you upload an uncompressed version of the track, and they will encode to OGG with a target LUFS. Classical music isn't impacted by this; it's loud rock, pop and EDM with aggressive limiting where the volume gets turned down. When people complain about the sound quality, it's largely junk masters handed over to Spotify, not the encoding process. Granted, if you _really_ try to listen to artifacts, they are usually in transients.