I use M40s and a VP150 across the front in my setup. The impedance of the VP150 is listed as 6 ohms. My receiver has an impedance switch from 8 ohms to 4-6 ohms. Even thoough the M40 mains list 8 ohm impedance, should I switch the receiver to 4-6? I've had no problems thus far, just wondering what's best or safest for both the receiver and the speakers.