I have always favored direct radiators for the rear surrounds. In movie mixes, the rear surrounds are very likely to have sound effects panned to them that are meant to be specific in their imaging, like an airplane flying over. A di-pole speaker would not present these sounds as intended.

The surround array that is used when films are being mixed is all monopole direct radiators. There are usually two on the sides for surround and two at the rear for rear surround.

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The Soundhound Theater