Smooth power response
Loudness perception in room is based on complete reverberant field not only on direct sound. So it's very related to loudspeakers power response. And that is why power response should be the same as on axis frequency response. With such power response the sound is good in any place of the room. And the speakers fully disappears acoustically. Just remember how much frequency response changes if you move around ordinary box loudspeaker in a room. And even if you don't walk around, the effect is also apparent if you turn the head.
Technically to get well controlled power response (constant directivity) is not an easy task. It can be made in several ways. The Bang & Olufsen Beolab 90 speakers do it in a very hi tech way with its 18 drivers in a speaker. I chose dipole because:
It is the easiest way. But even that it's not perfect. Polar response starts to drifts away from perfect before 10kHz in Pic.1, because of high frequency driver size 60mmx80mm. Which is big for its wavelenght. Problem is solvable by having less than 30mmx40mm. While good quality such small dipole driver is not available for now i work with what i can get. This is in a future upgrades list.
Dipole has 4.8dB directivity index. So direct sound to reflected sound ratio is increased. So room influence is minimised. It does't mean that monopole sounds bad in room or doesn't have its place in audio, but with dipole you deliver more recorded information to the listener ears.
Dipole has practically no baffle at all. This enables to get rid of other unwanted problems related to box or other open baffle speakers with big baffles.
And the last but not the least - it just sounds stunning.
Pic.1 Sonogram. Free field measurement. Frequency dependant windowing with 1/6.9 octave resolution. Note it is 20Hz - 20kHz -90 to +90 degrees sonogram.