Does Spotify stream a lower bitrate to PCs than mobile devices? It sounds like muddy compressed garbage through my PC, and nearly lossless from my phone using the same DACs.

I don't think it's RF noise from the PC, or AC noise. It's dead silent when nothing is playing. FLAC files sound the same on both. I have a gigabit connection so it's not a bandwidth problem. I don't have the issue with Pandora and it's supposedly lower quality.