Could it possibly be that the speaker levels are not set correctly? We have the 7-4 with a pair of Beolab 9s and rear speakers and are very happy with it and do find voices to be very clear. We did bump up the level for the Beolab 7-4 just a bit above equal in sound to the fronts. You can do it by ear although we had a friend bring a sound meter over to measure exactly. Changing the settings to rear would make things worse as you will only get occasional sound, surround sound, from the rears.
Beovision 7-55 with Beolab 7-4, 9s and 4000s
Beovision 10-40 with Beolab 1s and 6000s
Beosound 1, 5, 2000, and 3000
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