Who said “ flat freq response “ is the best?

@elizabeth ,  I think you are absolutely, and without any doubt in my mind, 1000% correct!!!

Easier said than done sometimes maybe...but not impossible (if you don't confine yourself only to the most immediate possible causes and are persistent). And that will pay ever bigger dividends the further one can take it - even if someone decides to take it past the point where they feel it may already be 'good enough' - always plenty of unrealized rewards to be mined there.

In a room no system is measuring flat you have to go to heroic efforts to even get close. Aiming for flat from a design perspective is so end user has a known base to work from still many designs do not aim for flat response adding more variables to system synergy. Since massive variables exist every audio system sounds and would measure differently depending on these variations.

So much to chime in on:

Loudness curves - Yes, this makes it important to listen to a speaker at your normal listening volumes, or get one that is flat and use a loudness compensation like Yamaha has or Dennon used to. A speaker tuned for 65 dB listening is going to have more bass and more treble than one designed for 90 dB. Some of the Dynaudio speakers are great examples of this.

Imaging - Wilson, among others, has taken advantage of using a dip at 2.4kHz or so which gives a perception of enhanced imaging, at the loss of some information. Not all wilsons, this seems to have fallen out of favor recently.

Detail - By using a ragged frequency response some speakers can appear to enhanced detail. B&W 802D and GE Triton 5 have done this, and bowled JA over. I can’t stand them, but I am not buying your speakers.

The GE Triton 1 offered a nasty sounding AMT with a very exaggerated upper octave. To me that was painful, but apparently older listeners love them.

My last point is, your hearing is unique and changes over time, so regardless of a technical ideal, or a particular reviewer's take, you need to buy speakers that are ideal for you. If you know you have hearing deficiencies in certain frequencies, maybe that is something you should look for when buying, speakers which naturally have more in that area. Nothing wrong with that. :) 

Going back to something @elizabeth pointed out:

The quality of treble is often a function of room acoustics. Try throwing pillows and blankets on the floor between and behind the speakers as an experiment. You may be awesomely surprised at how much this enhances sound, despite the first reflection mantras dominating the topic of room acoustics. 

Best,

E