The FMV stack — bass, middle, and treble in a passive RC network — is the tone stack that defined both American and British lead tone. It is lossy by nature, so it is fed from a cathode follower: a triode wired for a low output impedance that drives the network cleanly without the preceding gain stage having to.
Fender introduced the arrangement on the tweed Bassman, and Marshall carried it straight into the first British lead amps — which is why the same three interacting controls, with their characteristic scooped midrange, sit at the heart of amps that otherwise sound very different. The cathode follower is what makes it practical: without that low-impedance drive the passive stack would load the signal down and the interaction between the controls would collapse.
That scooped, aggressive voice is the through-line of this group, and it is a direct circuit ancestry — see how the lineage runs from the Bassman into the British amps. The archive also keeps the simpler relatives: the two-knob cathode-follower treble/bass stack and the plain treble/bass stack.