Very simplified. I assume a typical stereo setup, two speakers and no subwoofer.
.1 The amplifier has two channels. The job of the amplifier is only to amplify the input signals: left input signal is amplified and sounds from the left speaker, right input signal is amplified and sent to right speaker. The amplifier should not add anything or modify the ...
Audio engineering uses decibels, because they are closer to how the human hearing perceives loudness: a double energy level is not twice as loud. A linear scale would require either a very large fader or be fiddly to tune towards the lower end.
If you're tuning your EQ by the numbers, you're doing it wrong. Use your ears! ;-)