I could be wrong, but viewing this as part of the recording chain: the microphone(s) pick up the sound waves and convert to an electrical representation, normally ~20-20KHz. From this point until the signal is reconverted back to sound waves, ideally that same flat 20-20KHz is maintained. As mentioned, most electronics are very flat (or can be); the microphone or any transducer won't be, but in general strives to be barring special circumstances. So, an amplifier or other electronic device that is not flat could be viewed as a tone control.