I'll answer what I can.
A dedicated Digital-to-Analog Convertor will take that load off the device, improving sound quality substantially.A lot of people talk about using an audio DAC. I am curious why people do this as opposed to just using the headphone jack? Is the audio quality that much better or is there another reason??
Never mind that the hardware in a standalone DAC will be superior to whatever your Nexus uses, be it software or inexpensive hardware.