Too answer your question in a very general sense -
Phone DAC - if you can, definitely move to a high end DAC over the one in your phone. Keep in mind it mpeg music is what the phone designers figured would be the most common format.
Bypass Preamp - if you don't need the controls and the external DAC has a volume control option, then you can easily bypass it.
Preamp DAC - some do, some don't. Check your owners manual to be sure.