• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Internal Wiring for Tube Amps

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.
😳 🙄 🙄 😲