Initially, the direct connection seemed more dynamic with more detail but the preamp added more warmth, less fatiguing and more realistic soundstage. Ears are the best judges.True, if the transparency/dynamics of a direct connection became fatiguing, then it's a component in the system that's causing this, not the direct connection itself.
And yes you can band-aid fix this by adding warmth with a preamp to give the warmth colouration in the right area. But they all sound different. So the "right area fix", it maybe hard to find the right preamp to do that with. Better off finding/changing the "fatiguing" component
Cheers George