what should I set my a/v rec to for minimum speaker impedence 4 or 6 ohms I am running older morduant-shorts ms50.

4OHMs will allow the sound to be louder, 6 would attenuate it. What is the impedance of your Morduant's? If they are 4OHM and the AVR is 4OHM stable you should be OK to avoid thermal run-off, (or shutdown).