I have heard For the electrical system to work efficiently, the impedance should be as per manufacturing specs. What that means in my mind, if it says no less than 4 ohms, then 4 ohms should be the target. Although, 8 ohms won't hurt it if it need be, especially if you have plenty of headroom, since volume loss is the main by product. It will not hurt you amp to run your speakers that way, but two speakers with different ohm ratings, one will sound louder than the other. You can run them separately on the stereo lines to help compensate for volume that way. It should not hurt your amp to go higher with the ohms, but it will not be as loud and clear at lower volume settings. I think what most people like to do is keep their amp in that ohm zone where it gets the best performance, tone and volume, but it won't fry it unless you are running it hard under 2 ohms for a while.