Green Mountain discusses power linearity on their site but I have no idea if they do anything better than anybody else.
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Billy Woodman believes it is very important. It is one of the main benefits of a dome mid range. The absence of an even power response in the mid range in most designs means that the much more dispersive bass and tweeter will tend to dominate the reverberant field and mask mid range detail...furthermore the sweetspot will be much narrower.
Your point is a valid one...most designers do not worry about this limitation of a cone mid range.... and, to be fair, an even power response is of less importance in near field applications than in far field applications.
Rule of thumb => if you have a cone mid range then try to sit directly in the sweetspot and close to the speakers.
I personally believe the power response is far more important than the on-axis response. The measured on-axis response only happens at a single point in space. The farther back your listening location, the more and more the power response dominates the perceived timbre. From about 5 feet on back, most of the sound that reaches your ears is reverberant sound.
In my opinion, good power response matters for two reasons: First, the ear derives timbre not only from the first arrival sound, but also from the reverberant sound. Live instruments give you a natural-sounding reverberant field, but very few loudspeakers do. One test is, walk out of the room and listen through the open doorway, with no line-of-sight to the speakers. If they still sound like live music from the next room, they're getting the reverberant field right.
The second reason getting the power response right matters in my opinion is, I have come to believe that the ear/brain system finds a significant spectral discrepancy between the first-arrival and reverberant sound to be fatiguing over time. I can go into more detail about this if you'd like.
Loudspeakers that do a good job with the power response include Maggies, SoundLabs, Gradient Revolution, Altec Model 14 and Model 19, JBL Model 4430, BBC-spec LS-3/5a, MBL Radialstrahler 101e, ESP, Mirage M-1 and M-2 and recent Omnispheres, Shahinians Obelisk and up, GedLee Summa, large Tannoys, corner horns by Klipsh or PiSpeakers or Classic Audio Reproductions (Hartsfield), Allison Acoustics, SP Tech, Linkwitz Orion, and my own Stormbringers and Jazz Modules. I'm sure there are many others that I can't think of offhand. As you can see, good power response can be accomplished with planars, direct radiators, and horns (or waveguides).
Our local audio society had a demo day at NHT yesterday. The NHT folks talked a lot about power response and how their XD system does a good job with it. To demonstrate, while the music was playing, they turned the satellites around to face the front wall. The result was much less change in overall presentation than you would normally Impressive. I think I'm sold on the importance of power response.