When you get a cd/dvd, it contains music which is recorded the way the sound engineer wanted it to sound, agreed?
And when one plays that cd etc, the listener should hear the music just the way the artist and sound engineer wanted it to sound, agreed?
What is the thing which makes the same cd sound the same on different Systems? There must be an index or a "reference" for this purpose, agreed?
Well that reference is called a reference curve my dear sir. All systems that play that reference curve should sound the same, agreed?
And to get a system to play that reference curve it is eq-ed to get to that curve. When talking about a "flat" or "linear" response it DOESN'T mean that the "equalizer" knobs or settings are flat, it means the system's response is flat, period.
Example
A speaker system played in one car/home shows a 5db boost @ 500hz and 2db boost @ say 10khz.
The same system when played in another car/home shows a 10db boost @ say 60hz and a peak of 4db @ 12.5khz
Now the equalizer in both instances was flat, but the response wasn't. Get my point?
To call these systems flat, the first situation requires a 5db cut 500hz and a 2db cut @ 10khz at the equalizer bands, either parametric or graphic
Similarlythe 2nd requires a 10db cut @ 60hz and 4db cut 12.5khz
I don't think i can explain this in simpler terms than these or anyone else for that matter. If still dont get it then i request you to research on it online sir.
Google in-car and in-home frequency response. I'm surprised i had to tell you this basic information regarding eq.
Get a reference curve on rta, you will hear each detail in the instrument as the recordist wanted it to. And to get a reference curve you need eq, period.
Cheers!