The reason it sounded great, given the setup you described, is most likely due to phase shift. This is often employed on systems to fake surround speakers on 3.1 setup's. You would lose this effect with a splitter or common source not designed to drive surround speaker layouts. You would be better off by utilizing the digital/optical outs on the Mac. Keep in mind, you won't find a whole lot of 5.1 audio content so you will find yourself only using that for movies.