Who said “ flat freq response “ is the best?

Speakers posting their frequency response often are challenged by measurement (for example, every speaker reviewed in Stereophile), with peaks and valleys here and there all over their range. This might not mean much if the designer listened to them, which is generally the case. Also the frequency response noted by manufacturers is often utterly incorrect...I have a pair of very coherent, great sounding Silverine Preludes, which list the low end as -3db at 38hz, when at that point they're more likely -10db at least. My Klipsch Heresy IIIs are rated to around 58hz and that's pretty much exactly right...my subs take over at that point so it's all good.

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 a bass hump was engineered in over the years simply to make bookshelf speakers seem more alive and warm. However, if it's not in there, and more accuracy is involved in the design, you can utilize subs to make up the difference. The bottom line with this stuff is for a speaker to deliver a coherent and reasonably accurate sound that listeners actually like.