WHAT WE MEAN by "FLAT OUT OF THE BOX"
In my opinion, you can look at a frequency response from two angles: technical way or psychoacoustic's way.
Former is based on mathematical models and empiric experiments that delivered equal loudness contours from which Fletcher and Munson's are the one mostly taught in schools, due to being one of the first.
The latter is a bit less scientific if you wish or at least less mathematical. It's based on research with users and compiling their feedback into a curve that gets revisited every once in a while.
Almost every company making a playback system will have its own contours that they trust.
In our case, HPS S4 and S4R were measured using a curve supplied by Mini DSP and smoothened with the psychoacoustic filter provided in REW software.
We did that to represent the flatness efficiently. You can find a detailed response here. The simplified version is also there, so less experienced engineers/producers and musicians can understand, what is the value, we're trying to bring to them with our product range.
COMPARISON of S4 S4R DT770 DT990 M50X
We researched the flatness of our headphones during development with our ambassadors.
So they got a prototype model S2, S3 or S4, depending on how far in development we were at the time of their application to the trials. We conducted interviews and collected information on what needs to be changed to achieve a true studio monitoring headphones. The model S4 is very close to what is desired or considered as a perfect response for the majority of engineers. We're continuing to innovate and develop new acoustics for the S5 model that will hopefully bring us even closer to our goal.
I hope this clears what we mean with our statement "Flat out of the box."
If you have something to add to this or have a question, write to email@example.com