Kenazfilan - about doubled up speakers for a bi-pole. Today I gave the configuration a whirl with 2 pairs of CS2.2s back to back. I am driving with a pair of Classe DR-9s and Benchmark AHB-2s both at 100 wpc/8, both bridged to mono. 4 amps into 4 speakers. The back pair is driven by a PS Stellar Pre with remote volume control to bring up the back channels to suit. At some point of about equal loudness back and front, the sound takes 3-D form and moves back between the speakers and wall. The effect is delightful and doesn't seem to harm image specificity. The bass especially becomes big and round and clean.
Since for the same volume level, each speaker is working half as hard, distortion is undoubtedly lower; and since the point source patterns are reinforcing each other front to back, a quasi hemispherical wave-front is being launched. For those who love omnidirectional launch, this is a wonderful way to get it.
For fun, I reversed polarity of the back speakers (a button on the PS remote) which produces a di-pole pattern like a planar speaker. Indeed I got the phasey boundary effect. Back in bi-pole, the speakers just sound bigger. I suspect my room might be easier than some for this use. My walls are soft with lots of diffusion and little in the way of standing waves because 2 walls are sonically fairly transparent into a much larger space. The coupled bass from 4 speakers allowed me to pull the speakers about 6' from the front wall to image like champs while maintaining plenty of bass.
Fun at the end of the day.
