I use Google Play Music with the files downloaded to the device. I'm using a Nexus 6, btw. I know it's just MP3, but should still be 320 kbps.
From what I understand, viper shouldn't be touching the audio files being sent to the HU anyway. If I disable it, it makes no difference in terms of quality.
Maybe I just need to play with the settings on the nex to get it sounding right, but left untouched the music sounds flat. No lows or highs. I'll download some flac files and see how that works as well.