I don't think you will have a problem. I believe that speakers are always changing the ohm that the amp sees anyway depending on what they are playing at the moment. The ohm rating on speakers is more of an average impedence. It's not absolute. Your 8 ohm speakers may dip down below 4 ohms on certain passages. As to whether that will cause any gain problems. I don't know. All of my speakers are 4 ohms and I haven't seen any problems. I would guess that the calibration procedure would take care of most of that.