Here is an excellent paper on biamplification explaining the distribution of power, why smaller amps can be used for the upper frequencies.http://www.hometheateraudiocontrol.com/FAQ_detail.asp?id=1666
"Crossovers and Biamplification"
also
http://www.hometheateraudiocontrol.com/FAQ_detail.asp?id=1667
"Crossover Networks from A to Linkwitz-Rielly"