I have a Yamaha TSX-15. It came with two speakers rated at 4 ohms. I have three questions: 1) Can I add two additional speakers simply by plugging the additional four wires into the existing terminals? 2) What if the additional speakers are rated at 8 ohms? and... 3) If this is not possible, is there any harm in using the 8 ohm speakers only? Thanks to any who are willing to reply...