I am finally completing my M&K S-150 5.1 system. I love the dipole sourrounds (SS-150s). I do want to upgrade the system to a 7.1 setup. The only problem is that I don't know what to use as the rear direct radiating speakers. Do they have to be 4 ohm speakers like the rest of the system? Can I get away with M&Ks select certified surrounds? Not sure.