As I mentioned below, SC is limited to 128 mp3. Put youtube in 720p and the audio is decent-bitrate AAC, pretty much transparent. This is SC's most glaring and easily-fixed problem.
The low quality is actually a boon to established artists, it gives people an incentive to buy the record rather than streaming it again and again. For unknown people trying to promote their sound, not so much.
Part of the problem is Google's stinginess over bandwidth. They keep hiding the stream quality option more and more (referring to Google Play Music as well as Youtube) and they are literally RUINING the music. I've had to put HOURS of manual effort into changing stream quality on Google's products.