I'm replacing the rear speakers in my car and the factory ones say 6 ohm. However its much easier to find 4 ohm. I'm looking at some cheap ones on amazon rated for 30rms, the original say 15w on the back so I assume that is the rms for the factory ones. Could this damage the radio (still going to be using the factory stereo) or should I be safe? I don't know anything about speakers.