Hi theaterdoc,
It's very room dependent, but in a 16 x 22 ft. room, adding the extra center rear speakers might deliver what I've come to term a "modest enhancement." Usually the extra channels are more effective in much larger rooms where they provide more even surround coverage. With QS4s or QS8s in your room, you may find (as I do) the sense of envelopment with the classic 5.1-channel setup, totally convincing.
Since 6.1 and 7.1 receivers aren't all that expensive these days, why not get one? And the 6.1 and 7.1 are not discrete formats (the center rear is a derived matrixed channel, except for dts for music). Still, some users report big differences in their installations. Like so much of multi-channel setup, it's very room-dependent.
By the way, Klipsch tends to build aggressive, fatiguing horn-loaded speakers (not neutral).
Regards,
