09-22-15: Bdp24
Having enough gain in a source component is not all that is necessary to get the best sound out of that component. The output stage of a good pre-amp is engineered to drive the input stage of a power amp, while any given DAC or phono stage may or may not be.
In a related topic, it is for the same reason that many listeners (no pun intended---see below ;-) find the sound of a passive pre-"amp" to lack something (see Art Dudley's column in the current Stereophile).
However, over these last years dedicated DAC/preamps have arisen that are neither comparable to stand-alone DAC's in the traditional sense nor passive preamps; instead their purpose being precisely to act as preamps in addition to being DAC's, and hereby successfully driving a power amp on their own - as a singular component. Not only are they effective in doing this, but they also rid you of extra sets of power cables and IC's with shorter signal paths to boot. Indeed this discussion seems to have become somewhat dated in light of the development in this field for some time now.