Can it be used as a Dac & Pre-amp at the same time?
Yes. It is a DAC, so as long as you are running a digital signal into it (to be converted into an analog signal), you are using its DAC functionality. You can also select from between a fixed level output (use it as a source component into a preamp), or you can select variable output and use it as a preamp. The volume knob is the one on the right side of the face. It also has an analog input for those that want to have an analog source component in their system as well.
Hope that helps.