Hi, I have few questions about my setup if I could get any help from you. I need some education here. My setup is as follows.\n I listen to music on my iPhone, streaming from Amazon HD\/Ultra HD Music Service (evaluating it and my setup). I connect my iPhone 6S+ to my AV receiver, Yamaha RX-V675, via a “lightning to USB” cable. And listen to front channel speakers, Polk S55 towers.\nQuestions:\n1) Am I using internal DAC of the Yamaha receiver? \n2) Would a better external DAC improve the quality of music? Or am I limited by the quality of Polk S55 speakers.\n3) If I would benefit from an external DAC, how do I connect it to my receiver (bypass internal Yamaha DAC) that has to drive my Polks. Are there any recommendations for the DAC ? I must use my iPhone unless I buy another integrated amp or AV receiver. Thanks.