I have to agree with ghdprentice and phantom_av: the vast majority of the time, a great (tube) pre-amp is essential.
It really depends on what you have. We have tried to forego a pre-amp and go direct from Streamer (Linn Selekt DSM w/Kalayst DAC) to our Pass Labs monos. And while there were gains in resolution, it was not the type of resolution you want to actually LISTEN to--at least for more than a song or two.
The tube pre-amp gives us all the resolution of the direct method--that is: everything we heard going direct was there with the pre-amp. But here's the the trick: it was the pre-amp's arrangement and emphases of the frequencies and detail that made it both (audiophile-terminology alert!) musical/dynamic and cohesive (properly packaged with sounds in proper relation to each other).
Perhaps if we had the MSB Select or dCS Vivaldi+, we would have different results. But I really do not think so.
The gains in going direct are unbridled detail. But is that what audio systems are designed to do? Maybe a stethoscope is. But a MUSIC system is supposed to trick our brains into believing (coupled with our willing disbelief) that we are sitting I front of the live performance.
And that requires a tonal and dimensional consideration that a direct connection is usually incapable of delivering.