Who said “ flat freq response “ is the best?

Handyman has it right check out The Fletcher Munson Equal Loudness article.
This is one reason you see a "loudness" switch or adjustment on some gear.
Frequency response changes with volume.

The performance space affects the sound on the way to the recorder. Your room affects the sound that comes to your ears.

Having amplification that has an output that is exactly the same as the input except for variable volume, just gives you the chance to hear something like what you would have experienced at the performance - except for your other gear and your listening room, which is unlikely to be totally passive.

But if you can at least eliminate one source of colouration, I'd view that as a positive move.

Speakers need to be "voiced," i.e. you have to listen to them. I talked to a speaker designer years ago who said that if you had the ability to adjust the various sonic parameters of a speaker from your listening spot (including cabinet materials, crossover settings, speaker materials), the results you would wind up with from simply listening to music would be far from flat. 

I think most music lovers and audiophiles can agree that there is no such thing as “flat” anything - especially speakers. Although, it is probably a good thing not to have prominant peaks or dips in a speakers’ response. That said, I believe that the key ingrediant to great sound is the room itself. Assuming your equipment is well matched, and you are at a certain price point, you can get more bang for buck in room tweaking than in component changes.
I have 2 systems that I listen to frequently.  The one with superior room acoustics sounds better, no matter which equipment I use in it.