Saying that tubes may be warmer and S.S. may be most of the times brighter dont push us way much along the road...Actually it does if you understand that this phenomena is entirely the result of distortion- the tubes make the lower harmonics which the transistors do not; thus resulting in tube amps sounding warm and solid state sounding bright, even though on the bench both measure flat.
This also give the designer access to what to do about it; if a solid state amp were made with the same distortion signature as a tube amp it would also sound like one. The relative output impedances of tubes and solid state is almost irrelevant since the ear has a tipping point where it emphasizes tonality induced by distortion over tonality induced by frequency response error. That is why a tube amp potentially can sound more neutral even though it might not be as flat on a certain speaker as a solid state amp.
This is not about tubes/solid state though, its about **distortion**. We've proven that if a solid state amp has a tube-like distortion signature it will very hard to tell it from a really good tube amp. While this might appear subjective (which is what these terms are about) in the end this is the sort of thing that's easy to measure these days as well.