If all is impedance correct between source and amp/s.
They sky's the limit, as no active preamp can equal the transparency/dynamics/lack of distortion, that direct source to amp/s feed can give.
Adding a preamp in this case only diminishes and colours these parameters.
I don't disagree on paper that any preamp adds "something". That something can be a good thing. I like to pick the mix of tubes that suits my listening taste. My Linear Tube Audio MZ2 when used as a preamp is extremely transparent and distortion free.
It adds a degree of 3D imaging that direct to amp alone can't match. By selecting the right tubes I've maximized dynamics and gotten the tonal balance I prefer. I've tried source direct to amp and it benefits having this extra "spice". On paper observations don't always relate to the best or preferred sound.