Rick, although even surround speakers should match the mains as closely as possible, using what you now have will certainly give you a workable result. My suggestion is to use the Heresys as mains with the Def Tech center. Then the present Def Tech mains should be used as the side surrounds and the present Def Tech surrounds as the back surrounds. There's no good reason to be thinking of instead hooking up the existing Def Techs in parallel as a side surround array and no reason to use only one center back speaker.

Different impedance ratings on the speakers cause no problem; the amplifier in the VSX-32 can provide the voltage and current that each needs at a given instant.

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Enjoy the music, not the equipment.