Most DACs do not use passive analog attenuators, the attenuation is done in the digital domain, by dropping bits. At medium to lower listening levels, they lose a lot of detail, and the sound quality suffers. A dedicated, active pre-amp has high quality output stages that are designed specifically for driving amplifiers. Sometimes, well designed passive pre-amps can drive some amps very well, but it can be hit-and-miss. You may want to look at a DAC like the Bricasti M3, which does have a good analog, variable pre-amp output.
DAC DIRECT IN TO AMPLIFIER OR TO PREAMPLIFIER
For the longest time, I believed that the best preamplifier is no preamplifier. Eliminating a component from the audio chain would yield less distortion & greater purity.
Recently, I have had reason to re-think my logic on the matter - and I am (I think) changing my mind. Better said, assuming that the preamplifier in the component you are using (in my case, a DAC) can produce 95% or greater quality sound compared to the preamplifier component, then no preamplifier is the best option. The 5% represents the (estimated) loss of fidelity in adding another set of interconnects.
That said, most DACs do not have an outstanding preamplifier built in. I think most have average passive attenuators, and the better DACs have active preamplifiers that are very good - but not as good as a quality preamplifier.
What are your thoughts?