Turn a power amp into a subwoofer amp? How?

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

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.....