09-05-11: Amfibius
There is no such thing as no preamp. A preamp is simply gain stage + volume control. Just because your DAC has a built-in gain stage and volume control, and you elect to connect it directly to your power amp, does not mean that you do not have a preamp in the circuit. You do - your preamp is built in to your DAC. If your DAC has an elaborate gain stage, then adding a preamp would quite concievably worsen the sound, because you would be adding additional circuitry.
you're thinking about *analog* volume control; digital volume control works differently (i'm not going to go through the detail of digital volume control here because it has been discussed many times before) and does not have the issue to which you are referring. as a clue, if you see a device that has fixed and variable outputs, then the volume control being implemented is most likely analog. with a digital volume control system, you would typically only have a single output (of course you might actually have both single ended and balanced outputs, but they would be carrying the same signal).