The Volkswoofer is a dual subwoofer with a "push" and a "pull" unlike most subs that just push. An amp that just pushes won't work with this old sub. You need to get a dual amp or replace the speaker.
I need an amp for mine and can't advise where to go to get one but I just started looking. M&K is back in business so maybe they can give some advise.
Phil Layer
October 2009