A ruler flat speaker response is neither desirable nor possible. In fact, few speakers are flatter than +/-2dB and the average has more peaks and valleys than that. If an electronic component measured like a speaker you’d be horrified.
I have listened to a few misguided attempts at flat speakers. One (that shall go unnamed to protect the guilty) was flat to +/-1dB from 40Hz to 16kHz. It sounded awful: Lifeless, bright, unengaging—the opposite of what you might expect.
I suspect part of the reason we don’t like the sound of flat is we are so used to the dips and bumps of our speakers that any deviation probably sounds wrong.
More interesting is the deliberate application of selective frequency dips. If we accept a speaker cannot be flat (because it is not technically possible with today’s drivers), then clever designers use this fact to their advantage. For example, speakers with good soundstage depth are slightly recessed in the area somewhere between 200Hz to 1kHz. This recessed area may be as little as 1/2dB or as much a 1.5dB.
Another area is 200Hz and below, where there is typically a bump-up in speakers with a full response: less so in thin sounding devices. Take a look at this simplistic measurement of the Wilson Watt Puppy III from Stereophile’s review of them. Note the anomalies.
The blue line is with the Puppy woofer, the red line just the WATT.
I am certainly not singling out the Wilsons (fine speakers) just to make the point that measured flat doesn’t always agree with our ears and our rooms.
Just because something measures closer to an ideal does not a musical sounding device make.