Those pad controls vary the amount negative feedback in the first gain stage.
You see , each 12at7 ( valve ) is comprised of two gain stages.
The first stage is where most of the voltage magnification/gain takes place...
The pad varies the amount of feedback from that gain stage back to the input.Due to the fact that the signal is 180 degrees out of phase in this part of the circuit , the post amplification signal subtley cancels out the original by a small amount....this is gain reduction.
Along with gain reduction , negative feedback also reduces the input impedance of the first gain stage as well as its output impedance.This interacts with the output impedance of the secondary of the input transformer ,, and consequently the sound scape.
It kind of sets up 'modes' or preset colour amounts by the virtue of the signal path.