Who said “ flat freq response “ is the best?

A flat frequency response means all frequencies are reproduced at the same amplitude. Even if that was somehow possible to achieve in a listening room, that would remotely resemble what live music sounds like. I think a better use of this measurement in as far as comparing speakers is concerned, is how wide of the frequency spectrum the speaker drivers can cover and not if there are ripples within that response.

This discussion begs this question: would two different speakers with identical frequency response curves sound the same in your room? I'm betting the answer is no.

"Try throwing pillows and blankets on the floor between and behind the speakers as an experiment. "

Don't forget the fake plastic plants behind the speakers...!