OT Subwhoofer amp

[--M]

    I have this old subwhoofer that produces excellent bass, but it
sounds bad. The bass sounds good but the subwhoofer is not powered it
filters the audio signal sent to the speakers and they loose a lot of
midrange. I was thinking of making a power amp for this subwhoofer then
run it off the preamp out from my stereo. How can I figure out what type
of amp I will need to make?

--M