Who said “ flat freq response “ is the best?

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.