Normally a DAC is designed to drive a high impedance load (like 10KΩ or greater).
And normally you want, generally speaking, a 10:1 ratio (or greater) between the input to output rated impedances.
And again normally you don't drive headphones from line level outputs, mostly due to the (in)ability of the line level amplifiers to deliver power (current).
And if you look at the impedance curve for the HD-800's you'll see that they dip well below 600Ω, which in effect is a reverse ratio relationship to the line level outputs from your dac.
Now having stated all of the above, will the headphones make music? Yes.
Will the electrical relationship be matched properly? No.
If you look at the specs for your dac, pay attention to the output impedance of the headphone output. "Output impedance 600 Ohms (XLR), 0,5 Ohm (RCA), 10 Ohms (headphones)"
As an experiment try plugging in your headphones into the 1/4" jack on the front (you'll need an adapter, or just use the stock cable) and pay attention to the sonic differences.
Power amps are designed to deliver the power to drive dynamic loads (headphones and speakers etc) where line level outputs are designed to drive non dynamic loads (inputs to power amps etc.).
JJ