Usually the sub-woofer has the amp in it eg logitech Z623 - $120 - http/www.newegg.com/Product/Product.aspx?Item=N82E16836121048&cm_re=logittech_speakers-_-36-121-048-_-Product
That's only the case with PC speakers, most subwoofers only have an amp for the subwoofer itself, not the other speakers.