I like “culprit.” 
The learning curve is steep.
There is no conversion taking place. Spotify is sending a lossy 44.1/16 PCM data stream. Your equipment is simply passing this on to the Bridge which is just an input of the DSD.
“Lossy” means the data stream does not contain all of the information of the recording’s original data file. Some of the data has been removed to make the file smaller and to make it possible to stream the file at a lower data rate. The file/data stream is still 44.1/16 PCM. It is a little like taking a suitcase and removing some of the clothes to make it lighter and easier to carry.