Not sure if it's really a problem of bluetooth streaming quality. Maybe it's a problem of power of the streaming ?
Does the "quality" of the audio playback differs from time to time ? like when you move your phone ? near the receiver ? or when it is fully charged ?
Can you (or have you) tried with another phone ?
Finally, have you tried audio/volume booster apps ? or equalizer (Viper/DSPManager/...) ?