This question is due to curiousity and not necessarily the recommended way to do things. The passive subwoofer currently connected to my Pioneer 5.1 receiver does not provide much in the way of subwoofer or bass frequencies. While connecting a true powered subwoofer would be best, can the receivers subwoofer output connect to a seperate amp that provides the power to some larger speakers? If so, which input would you connect to on the amp?

Yes, you could, you would use the high level input on the amp. I think more people do it with their rear speakers into the high level inputs (they pass through).