I also will stand corrected that an active preamp may help if, the source (dac/cdp) has a power-supply problem in it's analogue stage(as PM states) not by just adding colouration, or as I have mentioned it's output stage/impedance and output voltage is not up to the task.
For myself if we have this kind of source with these types of problems that has a VC, I can't think of it being hi-end or been designed right, better to me to change it, mod it for something that is.
How can an audiophile live with his/her source knowing the analogue power-supply stage is not doing it's job (as PM states)
PM... Imagine we have a DAC with an identical output circuit to that of a preamplifier. How would this respond driving a power amplifier directly? Theoretically as well as a preamp and, perhaps, better because we haven’t another component in the mix. But here’s something you may not have thought about.
DACs are significantly more sensitive to power supply changes and noises than preamps. When an output stage struggles to drive a complex load, it is the power supply feeding its output stage that sees these changes. If this occurs in a preamp, it has little effect. But that same situation, when applied to a DAC, has very different results indeed. Small changes in power supplies have big impacts on sound quality–especially jitter.
So this is one reason, and there are more, some preamps can help a DAC.PM
Cheers George