Who said “ flat freq response “ is the best?

Human studies.  
 
If your gear measures perfectly flat on-axis in an anechoic chamber, and it has ideal off-axis (steadily and evenly sloping the further you go), then it will sound good in a “typical room”.   
  
Now, the room does play a large role, if you are in an untreated room that has tile floors and large windows, you may want a speaker that has a recessed treble.

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.