Interesting discussion. One topic I haven't seen mentioned is how the "Line Level" of a DAC or preamp output affects the equation. When I was young, I was taught that the main purpose of a preamp, assuming you don't want to color the sound, is to receive an input signal and output an uncolored signal to the power amp at "Line Level". So, in theory, if you have a good DAC (with a good volume control and analog output section) which provides an analog output at the same Line Level as a given preamp, how could the preamp do anything BUT add coloration? But, if you look up Line Level on wikipedia (https://en.wikipedia.org/wiki/Line_level), you'll see it's not quite that simple.
To use a specific example I'm familiar with, the Gustard x26-Pro is reputed to be a pretty good DAC, and some of the posts on this site have said that it's analog output section is a good one. Yet others will say that feeding its output directly into a high quality amp provides noticeably poorer sound than using it with a good, uncolored SS preamp, such as a Benchmark HPA4. Why would that be? I don't know the answer, but I'd bet it has something to do with the DAC's analog output being unable to match the Line Level of the output of the HPA4. This doesn't make sense to me if the DAC truly does have a good output section, but I trust the people who tell me it matters to them. If anyone would like to weigh in on this, I'd love to learn more.
To use a specific example I'm familiar with, the Gustard x26-Pro is reputed to be a pretty good DAC, and some of the posts on this site have said that it's analog output section is a good one. Yet others will say that feeding its output directly into a high quality amp provides noticeably poorer sound than using it with a good, uncolored SS preamp, such as a Benchmark HPA4. Why would that be? I don't know the answer, but I'd bet it has something to do with the DAC's analog output being unable to match the Line Level of the output of the HPA4. This doesn't make sense to me if the DAC truly does have a good output section, but I trust the people who tell me it matters to them. If anyone would like to weigh in on this, I'd love to learn more.