The original D100 has an output voltage of 1V. The Mk2 has an output voltage of 2V, which is the typical RCA output voltage.
One thing to consider is that your amplifier input sensitivity may be around 1V, in which case the additional gain wouldn't be of use to you unless you really like to push your amp past its operational range. Which is likely to dramatically raise THD.
Even if your amplifier's input sensitivity is 2V or higher, THD may be much higher at that voltage. My reference amp's THD+N starts to dramatically increase somewhere around 1.5V.
Of course, your pre-amp will be used to increase or decrease the DAC's signal voltage anyway.