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.
anyone? recommendations on powering a passive sub?
Dklap- The Hsu amp/sub combo must have some kind of feedback or servo control, otherwise, amp would not make a difference. You might try looking for an NHT mono sub amp, they have variable crossover and generally are pretty cheap used. Also maybe an NAD that you can bridge to get higher power, if you are willing to live with the fixed crossover from your receiver. However, Hsu sub/amp combo has had great press. Might be worth the $.