I'll do my best to explain this and without making too long. When setting up HT with a sub, your receiver allows you to "crossover" the low frequencies to your subwoofer. 1) It does a better job at it and of course goes lower than your speakers and 2)The receiver doesn't have to work as hard. This is what "sub out" is. If your crossover is set to lets say 80hz, all signals below 80hz will be sent to the subwoofer.
Your receiver gives several options for setting this up. (and there are many opinions how to) Usually you have the option of setting your speakers to small or large. I believe most opinions are to set your speakers to small for HT. (No matter what size your speakers really are) Of course when set to small, more bass signals will go to the sub. I have found I prefer my front speakers set to large for music. As well as listen to on only the front two vs all five. It sounds fuller to me. Well, my receiver at least and suspect most, when set to 2 speaker stereo for music (with front speakers set to large) even when set to "sub out", for whatever reason, it does not use the subwoofer at the crossover point. It isn't a real big deal, but would be nice for my personal preference. That leaves the options of having to set my front speakers to small to take advantage of that crossover point or set my receiver to send the low freq's to both the fronts and the sub. at the same time. Which I prefer neither over "front speakers set to large with sub out"
I hope this it explains it well enough. If any one can do it better, please do.
