An audio-friend of mine has always found it better not to use a separate hardware preamp in his setups, especially since his latest DAC/preamp (the Danish developed/manufactored Blue Cheese Audio Roquefort
http://www.studiosound.dk/cddac/roquefort/) sports a dedicated preamp section with digital volume attenuation. The separate preamps he's tested against it have all failed to deliver equally overall, until a very expensive preamp entered the setup in the form of a Belles LA-01 (driving his Belles SA-100 poweramp). While the inclusion of this hardware preamp to his ears doesn't necessarily translate into a win-win sonic scenario in all respects compared to the stand-alone BCA DAC, he's smitten especially by the added sense of "drive, dynamics and transient abilities - as if the existing components are harnessed into a fuller, better controlled potential in many respects," as he'd more or less put it. This is an intesting observation to me, also insofar it would take such an expensive preamp to finally turn it into (again, in some respects) an even more satisfying sonic experience.
This example - among others, actually - tells me that separate hardware preamps are a potential blessing in some vital respects, but are at same time an added component in the audio chain where many variables combine to make it a challenge for it not to impose too much of a character of its own (read: the challenge of transparency, if you will). I guess for some the above mentioned traits coming in the wake of a hardware preamp overshadow an added layer of coloring/character, where it might be more pronounced, whereas others (like me, for instance) would find it a nuisance - depending of course not only on taste, but also and not least the setup where these evaluations are made.
The motivation behind above mentioned friend trying out a hardware preamp in his setup was essentially due to the planned investment of a turntable, one might add. To me, with a digital source only and a very successful mating of DAC and poweramp direct-coupling, the inclusion of a hardware preamp would have to be so utterly convincing (not least in light of its expected severe cost for it to make a real difference) to have me forget the other areas where the same or less amount of money could make a difference. I've heard many preamp-based setups, and most of them to my ears truly lack the coherency, truth of tone, and snap and power found through my own setup - using no separate hardware preamp. A hardware preamp is not necessarily a sonic blessing in and by itself, and a preamp-less setup is not necessarily marred by what is so generally found in above postings. Just saying..
