You are quoting Linkwitz out of context in order to suit your narrow understanding that loudspeakers should be flat. Nowhere does he mention that this only works with dipoles, etc. As a matter of fact, his active crossover (kits, boards, finished products, etc.) has a permanent undefeatable 3kHz dip, universal for any system, situation, scenario, etc.
Regardless of how anyone would like to spin this, it all goes back to the equal loudness curve. For a speaker to sound flat, it has to be EQed. We've all been listening for way too long with this extra bite in the lower treble and have become accustomed to it. We blame the hardness and unnaturalness on bad recording, compressed audio, but we fail to see what the real issue(s) is.
BBC
Linkwitz
Audyssey
B&W
All wholeheartedly endorse this theory. That's a lot of sound engineers and pioneers.