It's not positive and negative. It's above and below a positive bias voltage (the dsPIC does not have a negative voltage supply). Just use a difference amplifier or differential amplifier with a gain of 1 if you want to re-reference the signal to ground rather than the bias point.
No, no block capacitors.The 33F DAC was designed to drive an audio amplifier. It does not have enough power to drive headphones directly since it is a data processing device, not a power device. You would use the bias output voltage of the DAC IN+ for the IC.
Just curious, were you using DC block capacitors when directly connecting the DAC to the headphones?
I dont think so? Sorry, I don't know...Is this a differential PWM output DAC? If so, I have several ways of helping you. Do you want a PWM amplifier (high efficiency but complex) or a simple linear amplifier (inefficient, lower power, but easy way to drive headphones)? Remember you should be lowpassing the signal if it is a PWM output.