I have had the same experience as the OP. In my case I have a somewhat vintage (late 90's) Krell KRC-2 pre. My theory is that the volume control quality likely has something to do with the superior sound in a pre. We do most of our listening at lower volumes where the line level signal is severely attenuated. A good pre and its volume control are optimized to insure that there is no deterioration in the sound at lower levels. A DAC is really a line level component and the volume control is not critical to its function. In fact, many DACs have digital volume controls that reduce the resolution as the volume is lowered.
DAC as pre amp
This post is just a brief for posterity sake in regards to the the significance of a good pre. As I have stated in other threads over the last few years , when the subject arise, I occasionally remove my pre from the chain and run my Wadia 321 as the pre. I always find my way back to my dedicated pre. I got some Quad Z-4s awhile back and after a recent post regarding the DAC direct question I decided to try again. This round the improvement was even more dramatic with the dedicated pre and the Quads. My modest system is good enough for my level of listening sophistication. but If I ever decided to upgrade a component in the future , the pre would be the unit I would explore. Another member stated that a good pre will ad a "richness" to the sound. I think that is exactly what I hear.