Turn a power amp into a subwoofer amp? How?

Will you use the amp 'bridged'? If so, can it handle the VC impedance of the new sub drivers?
Is it a DVC sub? If it ia a pair of 4ohm coils, you may want to run 1 to each channel of the amp. OR, wire them in series, especially if the amp won't drive low impedance loads well.

Unless you are DeadSet on a DIY, you might be better off doing a 'google' for 'plate amp'. They are inexpensive and have all the functionality you need, in one package.

I have an old Carver m-400t, 'The Cube'. That was my plan, too, but it gets out of hand quickly.....

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

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

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.