If your DAC or power amp has a volume control, then you don't need a preamp. Having said that, I haven't yet heard a DAC that doesn't incorporate some kind of line amp that sounds as good as a DAC going through a quality active preamp. Typically, DAC-to-power amp gives greater clarity but stifles dynamics and gives flabby bass.
However, I may not understand your question properly. What volume control are you talking about?
Joel.
