Alright, we all know that preamp and power tubes will give a different character to your amp. However, what many do not know is that changes in the amp's tonal character due to a tube change is subtle at best.
To expound on this idea, think of an amp like a cake, and its tube as icing.
If you get a chocolate amp, it will always taste like a chocolate amp. If you get a vanilla amp, it will always taste like a vanilla amp. Changing your icing flavour is not gonna change your chocolate amp into a swiss roll.
However, if you get fresh cream tubes, it will give a slight subtle change in taste to your vanilla amp, than if you used butter cream tubes. The taste of your amp will always be a vanilla amp, however, the fresh cream will be lighter, and the butter cream will be smoother.
To be continued... the difference of preamp and power tube changes... making reference to the cream (double full on cream vs single cream) on the cake.