Since the tube sound is mostly attributed to its high output impedance (which is why they are described as warm, as most speakers are ported). So, a tube amp is magnitudes more influential in coloration than a tube preamp.This statement is incorrect.
'Tube' sound comes from a lack of higher ordered harmonic distortion. Quite often tube sound comes from more 2nd harmonic distortion as well. Combined, you get smoother sound on top because the ear perceives higher ordered harmonics as harshness and brightness (the transistor 'sound') and a richer sound on account of the 2nd (if present).
Its possible to design tube gear to not make the 2nd harmonic (by making the circuit fully differential and balanced).
The higher output impedance that is common with tubes has little to do with this.
In a nutshell, because they make less higher ordered harmonic distortion, its easier to build tube circuits that more closely adhere to the human ear/brain hearing perceptual rules since it is distortion that causes transistor circuits to be brighter. Of course its a bit more complicated than that, but if you want what the 'tube sound' is all about then you're better off with both a tube amp and preamp.