I don't know that it's all that complex. Setting the speakers to small keeps your reciever from sending the low end to them. This takes strain off the reciever because it doesn't have to amplify that portion of the signal. Setting the crossover has a similar effect in that it sets the upper threshold of what will go to the sub. Choosing the proper placement of the sub for your room is done because bass is very finicky and travels much better through dense material than air. Thus you can get very different levels of bass without changing your setting just by moving the sub around.

[black]-"The further we go and older we grow, the more we know, the less we show."[/black]