Turn a power amp into a subwoofer amp? How?

just hook your receiver or processors subwoofer preamp output to one of the inputs on your 2 channel amp and then run the corresponding speaker wire from your amp to you subwoofer's binding posts. the receiver/processor should be able to handle the crossover point.

If you want to go DIY, have a look at http://www.marchandelec.com/ .

You'll want to make certain that the input impedence of the two amps - the one for your main speakers, and, the one for the subwoofers - is roughly the same otherwise the volume output of the speakers/subs will be unbalanced - i.e. the main speakers may be louder than the sub, or visa versa