660 Headset sound

[Prosnap]

I bought a Zumo 660 hoping to be able to hear some tunes through my Bluetooth Sena SMH10R.

All sounds fine while playing music through the 660's internal speaker. It all goes to pot as soon as I try connecting any type of headset .... Wired or Bluetooth. It all sounds very tinny and hissy ...

Any ideas?