Deep in the menus of the Rx400h there are some settings which can improve sound quality of bluetooth-kit...
Have a look there :) (similar) LINK
Bluetooth quality settings (Rx400h):
- Ingnition on
- Button "Display"
- Touchscreen top left and button left, touche several times
- Service Menu
- Button "Menu" (top right)
- Bluetooth quality
- send and receive --> +2
- to exit the service menu switch ignition off
Sorry, I translated it very fast from EuroLOC...