Comment regarding tube rolling:
The amp is 'immune' to tube rolling, as far as the tubes that are substituted have exactly the same curves.
So in effect the amp is not at all immune to tube rolling, since no two tubes in existance actually have physically identical curves!
I'm just being silly here. If you get two high quality tubes of the same type, say Tesla or military Soviet, that come from the same factory, same decade, they will have 97 to 99.5 % same curves.
Now take a Tesla high quality tube from the early 1960's, and a crappy RCA tube from the late 1970's... Yes they will sound different, because they have very different build quality and the curves WILL BE different.
But, within reasonable limits, i.e. using similar quality tubes, the differences will be almost nonexistent.
If some tube sounds "more top heavy" or "clearer" that is because it has a different inner capacitance between the control grid and the anode.
Generally more pronounced in triodes, since the 'anode' in a triode connected tetrode or pentode is actually a grid, which has nonexistent capacitance compared to a solid anode of a triode. So, EL81 and EL36, while clearly having differing inner geometry (just look at the elements with your eyes), sound much more similar, because the g1 to g2 capacitance is in both cases similar, nonexistent.
Of course capacitance affects 'tone', linearity is another matter. But those TV tubes are all superbly linear. Also superb in durability, they were made to withstand huge repeating current pulses without melting.