Roon digital volume control is fine, achieving the the same function as an analog premaplifier, but in the digital domain.
The output signal is multiplied by a constant attenuation factor. A reduction of the signal of a factor 2 (50% of the original corresponds to -6dB, and one bit in the digital domain. A reduction of -18dB seems a lot but is “only” 3 bits “lost” on a 24bit stream (remember that CD quality is 16bits…).
It seems that Leedh has implemented a way to attenuate the digital signal in a way that audiophiles like it more than a simple proportional reduction of the signal. The only think I could think of is a form of implementation of equal-loudness curves, somehow boosting lower frequencies at low volume to compensate for the non-lineaity of our psycho-acoustic perception of frequencies. But this has been done many times (remember this “loudness” button on your dad’s amp…).