Paul; Here is my most recent experience on topic:
I used an Audio Research SP15 as my preamp, an entry level DAC (Suncoz) to smooth my CD and hard drive inputs. I was dissatisfied.
I bought a PS Audio Perfect Wave DAC/streamer which is also a fine quality preamp. In fact Paul McGowan touts it as a substitute for the necessity of a preamp in some of his daily videos. I was excited about the front end of my system being a state of the art source / DAC and preamp. One less component, 3 fewer tubes, a lit less heat, etc.
The CD sound was fantastic- truly revolutionary. The streaming sound was meh most of the time but wonderful when the material was well mixed and of a high quality uncompressed nature. I had achieved the impossible- tube sound with an all in one front end. I was ecstatic..until:.
Poor volume control. The PS Audio preamp just couldn't give a strong enough signal to my amps to make a loud DB presentation. Full volume was about the same as loud talking. It sounded fantastic but I couldn't push it to the level my speakers and amps would support- and some music needs to be played LOUD!
Given my particular components my conclusion is this: With a high end preamp a first quality DAC is improved- not sonically per se but in overall control of the sound.