When the preamp and the DAC both have volume controls, only one of them should actually be doing the work.
This question comes up more often than you'd think. Many of us have a preamplifier and a DAC, both have volume controls — so where should each one be set? The instinct most of us have is to turn one down and the other up to balance them out, splitting the work between them somehow.
That instinct feels reasonable and is almost always wrong. The right answer is to do all the level control at the preamp and run the DAC wide open.
The reason is straightforward once you think about what each box was designed to do. The preamplifier is built specifically to handle low-level signal manipulation cleanly. Its volume control sits at exactly the right point in the signal chain, with the right impedance characteristics, to attenuate without adding meaningful noise or distortion. The DAC is designed to do one thing — output a line-level signal and make it big enough to drive your preamp — and any attenuation built into its output stage is a convenience feature, not a sonically optimized one.*
*There are some exceptions, like the PMG 512 DAC that has a true analog stepped attentuator at its output and the same sort of output stage as the PMG Preamplifier. That said, the addition of the preamplifier still blows away the DAC straight into the power amp.
One volume control per signal path. Pick the right one and let it work — every other knob in the chain should be wide open and out of the way.
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