Same 320kbps mp3 file transferred from my laptop to my phone SD card & to my Zumo 550 SD card. Sounds shite played from the Zumo, sounds great played from my phone (Xperia M4 Aqua). I don't know why, but it does. Maybe later/newer Zumo are better?
I could be wrong, but I don't think the 550 supports A2DP (the Bluetooth protocol for decent quality audio) so it's playing the music via the 'headset' profile (which is great for speech, but pants for almost anything else). I think A2DP support was introduced on the 660.