powering a passive sub...

looking to buy the HSU TN1220HO, but cannot afford the separate amp offered by hsu. what factors do i consider in buying a separate amp? at the least, i figure i need gain/level control to ensure the sub will match my system. receiver has built in cross over (fixed at 80 hrz). Dr. Hsu tells me if i don't use his amp, response will only be flat to 40 hrz instead of 15 hrz if i use his amp. not sure why...anyway, suggestions on powering this passive sub would be appreciated.