Who said “ flat freq response “ is the best?

Erik,

I second your thoughts on experimentting with room acoustics.

I designed my listening room with an acoustician so it’s a good sounding room.
But I also have some leeway in adjusting the reflectivity af various points.  I have a shag rug (yeah, baby!) and the ceiling is a drop down structure covered in felt with various portions of heavy absorption areas hidden behind.  So just those two elements alone tend to reduce a lot of room reflection hash etc for a tmbrally smooth sound.

But I also have a variety of curtains gatherered in the room corners that I can pull across any specific area of the wall to kill other upper frequency reflection points, or alternatively liven the sound by allowing more room reflection.   It’s great having this flexibility (and a recently added diffuser has increased the control over my room acoustics).

As to your suggestion for people to try dampening behind the speakers I can attest to that as well.  Several feet behind my main stereo speakers is a wall sized projection screen with a 4 way velvet masking system.  This means I can make the back wall behind the speakers very reflective (velvet curtains wide open revealing reflective screen surface) or completely covered in velvet, or anything in between.  Only adjusting the reflectivity of the back wall really has an effect on the room sound.  Too much and it becomes too dead.  But a judicious screen size setting behind my speakers leaves a really nice  balance.   The more I close the velvet the more the ambience tone and reverb comes only from the recording.  Opening the curtains makes the sound more open and lively. 

kosst_amojan
I completely agree with Elizabeth and Erik. It's real easy to get nasty treble when it's bouncing all over a reflective room and that jazz needs tamed.

>>>>You don’t know the half of it.

There are a few subjects being discussed here: 
Flat response from Equipment and speakers
Flat In room response. 
If you think about it,  when the recording is done,  are they done in perfect rooms?  Carnegie Hall,  Studios, Restaurants, Church's.  Every recording will reflect the room that it was recorded in,  then our room has an effect again on what we hear.  The only way to hear the recording as it was done is with a flat room.  Ideally, all recordings would be done in a perfect anechoic chamber and our rooms would be the same, but its tough, shoving a symphony in a chamber. 
Next, for Equipment,  I do believe that it is important for our equipment to be producing a flat response.  That is the only way to faithfully reproduce the recording, assuming that we can have a perfect room.  I believe that it was @kalali that posed the question, would 2 different speakers with identical response curves sound the same in the same room.  I can answer that, I have built many speakers and have come EXTREMELY close in duplicating response curves, but I can tell you for a fact, not opinion that different driver materials sound different and on crossovers, even using the same slopes and same type of compensations or no compensation that crossover parts still sound differently from each other. 
Overall, my contention is Absolutely,  you need to come as close as possible to achieving a flat response curve on your electronics,  in your speakers and in your room.  If that is achieved.  It is then that you get as close as possible to creating that all allusive "Live" sound that we all work so hard to achieve.  Plus when things are acoustically flat, it does help hear so many other design presentation differences in amps, pre's, cables etc. 

Look around; you'll see that even ruler flat response bears little correlation between the perceived quality/acceptance of a speaker by the listener. 

I have handled enough speakers which deviated in some respects which others might consider unacceptable, yet they can be made to sound beguiling. People get hung up on the search for flat response, just as they� get hung up on the search for "The One," the perfect speaker. Imo, fairly fruitless efforts. 

I have heard plenty of systems which were purportedly "dialed in" to be flat response. Meh; they were pretty "flat" all right. 

To this point, no one has mentioned phase.

Minimum phase error trumps small deviations in frequency response in contribution to a realistic and non fatiguing presentation six ways to Sunday.

It is a great failing of the audio press that phase has been largely ignored for the half century since Richard Heyser's seminal work.