a) A solid state amp that is well designed (read like 90% or more of them) will add no AUDIBLE signature of it's own. I agree with this.

b) Tube amps do something "magical" to the music, well some of them do, and make it sound more involving. I agree with this too. They do this by adding sonic signatures of their own, best measured by increased euphonic distortion and frequency aberrations that make "voices clearer" or "instruments better separated". Often, the effect is subtle.
For example, if you heard the old zen amp I had (decware zen se-84C), in my opinion it was a thinnish sounding amp, but it certainly made voices (esp. women;s voices) sound better. I have heard Wayne Parham remark that a couple of times, spontaneously, when we switched amps and changed noting else, keeping volume control approx constant.
Again, in my opinion, that decware zen adds its own sonic signature.
So, I agree with b) as well (note that neither a nor b are mutually exclusive).

Now, we need to ask ourselves: what is my goal? If it is ti get tehcleanest possible reporduction (HI FIDELITY), then one approach is to make EACH component in the signal chain add NO sonic signature. Soi, if I did that, I would have a cd player, very small interconnects, a solid state amp, shortest possible speaker wires with minimal LCR, and then...ahh the speakers: These are hardest to control. But if I could, I'd buy the speaker with the FLATTEST anechoic freq curve, the lowest possible distortion, and then treat my room so I got all of this at my listening position. IF I did all of this, then I would get the true recording, and all my music quality would depend on the quality of the recording.

The SECOND approach is: Buy imperfect components that somehow "mesh" together, often by cancelling out each other, and adding euphonic distortions, etc, and somehow make the whole sound musical. Often, they may actually make poorly recorded material sound more "natural" and also counter room issues.
I believe ALL tube amp owners follow this approach (whether they admit to it or realize it or not)! It's fun. and can result in suprprising good sound, not to mention a collection of fun TUBE amps.

As one of my friends said, it beats chasing women, drinking & gambling, a a clean hobby.

SO it;s really a matter of choice. I hope these comments can frame a debate on this issue, though there may be NO DEBATE: some folk will espouse the first, clinical approach, described above, while others will follow the second "muddly" approach.
The discord arises when both camps claim superiority in achieving high fidelity: in my opinion the first camp is better able to achieve hi fidelity, but you gotta ask yourself, how muhc fun is tat verus messing with tubes and vintage/single drivers?

-akhilesh