Earlier Amplifier designs had primitive tone controls ( called "tone stacks") much before the invention of the Baxandall
and the parametric / graphic equalisers.
Changing the tone control settings changed the volume
and changing the Bass control also changed the Treble response and vice versa !!
Many tone stacks had a mid frequency hump or depression.
However, guitarists got used to these primitive tone controls and we all grew up listening to the tone created by these
primitive early attempts to shape tone.
Here we must mention Duncan's Tone Stack Calculator
that allows engineers to see in real time the effect of the tone controls when arranged a la Marshall, Fender, Vox and other
tube Amp manufacturers.
A normal Amp configuration has a preamp stage, the Tone Stack, some more gain stages and then the power stages.
When the tube amp is running at low gains, most of the distortion is caused by the preamp stages before the Tone Stack.
The Tone Stack therefore acts as a post-distortion equalizer.
However when we start running the tube amp harder, we start seeing clipping of the other gain stages and power stages
after the Tone Stack. It starts acting as a pre- distortion equaliser !!!
Indeed, placement of the Tone stack in an Amplifier Model is crucial in attaining the desired sound.
Fender's Cyber series of Modelling Amps actually reconfigure themselves based on the Amp being modelled. The main changes
are the tone controls are placed before or after the tubes exactly in the same way as the original Amp being modelled. This
technology was important enough to patent