Personally, I don't like the sound of solid state amps or preamps. I think it's due to the prevalence of odd order harmonics. It's a matter of personal preference for sure.
If the signal from the preamp emphasizes the odd order overtones, then it stands to reason that the amp will amplify those overtones.
So, my answer is that tube preamp and amp are equally necessary.
If the signal from the preamp emphasizes the odd order overtones, then it stands to reason that the amp will amplify those overtones.
So, my answer is that tube preamp and amp are equally necessary.