for amp circuit, they have their own tonestack, which depending on the interaction between the 3 knobs, to boost or cut certain frequency. One of the more known circuit which some amps have for the tone control, is known as baxandall tone control. Google for it if wanna read more
without knowing the frequency range over the tone circuit(bass/mids/ treble)works, its hard to say whether the tweaking of what you have, will actually affect over the range of the low mids you wanna