A good preamp will have very low distortion at the levels needed to drive a power amp and wider bandwidth than the source.
The main things that a good preamp brings to the table is a better volume control than the ones found in most DACs (there isn't a good way to do volume control in the digital domain) plus the ability to drive the interconnect cable to the amplifier properly, so as to minimize the effects of that cable. I've yet to see a DAC that deals effectively with either of these issues.
We sell a lot of our line stages to digital-only people- they run the DAC volume all the way up so as to minimize the the damage done by that control and use the preamp volume instead. The preamp then can drive long cables where their DAC can't. In this way they report that the use of a good line stage is more transparent and more musical- more like real music.
Note my use of the word 'good' above; many line stages are not up to the fairly simple tasks I outlined above, and so do damage rather than help.
The main things that a good preamp brings to the table is a better volume control than the ones found in most DACs (there isn't a good way to do volume control in the digital domain) plus the ability to drive the interconnect cable to the amplifier properly, so as to minimize the effects of that cable. I've yet to see a DAC that deals effectively with either of these issues.
We sell a lot of our line stages to digital-only people- they run the DAC volume all the way up so as to minimize the the damage done by that control and use the preamp volume instead. The preamp then can drive long cables where their DAC can't. In this way they report that the use of a good line stage is more transparent and more musical- more like real music.
Note my use of the word 'good' above; many line stages are not up to the fairly simple tasks I outlined above, and so do damage rather than help.