many current-production tube components have circuits which are not tube sensitive. i have encountered many examples of this phenomenon. in such cases, replacing stock tubes is not beneficial.Going from one tube to another does not guarantee an audibly detected change. The two tubes could have very similar sonic properties. Or the overall system is dimensionally flat to not allow for one tube's refinement to shine over the other.
If you go to the extreme and try for example, a premium Valvo or Amperex pinched waist tube, and compare it to something like Sovtek or Electro-Harmonix, even the most compromised tube design will show this difference instantly. The comparison may result in nearly the same tonality but dynamic contrasts, ambiance and decays of the NOS are significant compared to the newer manufactured tubes. And no, I am not talking about "warmth". The newer stock tubes also tend to sound forward and fatiguing (edgy) compared to the far more smooth and natural result of the NOS tubes.
If the signal passes through a tube, it WILL affect it's sound. There's no such thing as a tubed circuit that is not "tube sensitive", in this regard(the tube being the most "active" component in the circuit).Agreed.