Can you clarify a little? If you are talking one device - and you're either going line-out or headphone out, then both are using the same DAC - the difference is in the amp. The line-out will be a constant signal which may be bypassing the internal amp.
If you can clarify the question - I'm sure someone can help.
I am talking about the Apogee Duet 2. It is a DAC used for recording and mattering albums. This is high quality for a small unit powered by the USB connection (it does have an optional external power supply)/ . I am thinking of using it for my headphone DAC. According to a representative from Apogee Digital, the headphones have its own DAC. According to them, what you get out of the headphone jack is "pretty close" to what you get on the line outs.