Ellen,

If I remember correctly from HT article, human ear cannot locate the source of sound raging from 40 HZ to 90 HZ (someone correct me if I’m wrong) thus using one equipment (subwoofer) to take over that duty for 5 or 7 speakers makes it economical in general. There are some disadvantages to it if you get really picky like this thread here: http://ubb.outlawaudio.com/ubb/Forum4/HTML/000250.html

For speaker design fundamentals, try these two sites. They should give you a good explanation of why different crossover matters so much.
http://www.speakerbuilder.net
http://www.loudspeakers101.com/Sound.htm