Using 5k5 Rears would be better than your current D5k's certainly. The D5k5`s would be better able to handle the lower ranges.

However, I think that the most obvious benefit would come from having sides in a 7.1 system. Many soundtracks are somewhat limited in the amount of information and frequency range that they put in the rears and more benefit can be had from expanding the soundstage

With the correct speaker placement and calibration a 7.1 system (with 4 by D5k's) will provide more obvious benefits and a more expansive soundstage than a 5.1 system with 2 D5k5's at the rear.

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