It’ll change the FR curve.
The way to think about it is that the output impedance of the DAC forms a divider with the input impedance of the amplifier,
If the output impedance is small relative to the input impedance you get close to a perfect voltage source, I.e. a pretty much the measured straight line response.
IF the output impedance were constant with frequency (which it won’t be) you’d just get a lower level signal to amplify, otherwise some portion of the output impedance vs frequency response curve is basically superimposed on the frequency response. So not a flat line.
You’d need to see the output impedance vs Frequency graph to know what that means, but for most output circuits you’ll most likely you’ll get a boost in bass frequencies and some loss at the upper end.
But I’d be concerned in this case that because the output is coming straight off the ladder you might also get significant change in output impedance with volume, and that would result in none linearity into low input impedance amps.
It’s generally why you want a high input impedance in an amplifier, but for most headphone amps that’s a direct result of the Pot that’s used and the higher the impedance the Pot the harder it is to manage RF noise, so the high SiNAD amps just assume low output impedance for the DAC.
It’s not the only DAC with this “issue”, some very highend DAC’s have similar or even higher output impedance, it just means you have to be careful with the pairing. In the speaker world where preamps are the norm, it’s almost never an issue,