Audio Systems: How to achieve optimum sound quality when using an Aux cord or Bluetooth

Question
I am using a Samsung Galaxy S4 and my sound system is the a Sony MCH-ECL99BT.

When listening to music using the bluetooth or aux cord from the speaker should I have the volume higher on my phone or on the actual audio system?

Answer
Good question.

It has to do with the sensitivity of the MCH-ECL99BT, its internal noise level, input sensitivity level and input overload point.

However, it is easy to check practically.

You want to avoid overloading the input level of the MCH-ECL99BT unit.  Therefore, to set it up, run the volume control maximum for the MCH-ECL99BT unit and the S4 start at zero output level.  Advance the output level slowly until the speakers are at maximum loudness you are desirous to achieve.  Listen carefully for audible distortion from the speakers.  You may near clipping distortion at an early level.  The waveform clipping distortion is very audible; you may hear it from the beginning or after you have advanced the speaker audio somewhat.  This test gives you a rough estimate of maximum input level the MCH-ECL99BT can take.  Now, reduce the volume control on the MCH-ECL99BT until the sound becomes clean.  Then reduce it a little further such that there is no audible distortion being produced by the MCH-ECL99BT and speakers.  This would be the optimum level seting.

If the output never reaches distortion level (before it runs out of audio power) it means the output voltage level of the S4 is insufficient to overdrive the input of the amp/speakers and you are okey to go.

Let me know if you have questions or thoughts.