Internal Wiring for Tube Amps

[Osvaldo de Banfield]

However, the reasoning is flawed: at the input, the resistance in shunted by the output of the preamp, usually a few K max., which pushes the low pass effect an order of magnitude...

I didn't mention where this resistance is. It may be a single unit (a grid leak for example) or a combination of multiple actual and parasitic resistances (input or output resistances, potentiometers, grid leak, attenuators, equalizing networks, etc.). I tryed to make a general example only. However I understand your concept and in fact makes my assumptions more realistic.
😳 🙄 🙄 😲