I would start off by investigating the condition of the driver. If everything seems okay i.e. the driver moves freely without dragging or scraping, the surround ( foam, cloth, etc... ) is in good shape, etc.. If that all checks out, I don't see a problem with using what you already have. I would personally pull all of the other drivers out out and seal the cabinet as best possible. While you are in there, install some decent wire and connect it directly from the input jacks ( whatever they may be on the cabinet ) to the terminals of the speaker itself. In other words, bypass any / all of the existing wiring and crossover parts.
Using this as a subwoofer is actually easier on the driver since it will only have to cover a smaller portion of the band that it used to have to cover as a full fledged woofer. How great it will work or how long it will hold up as a "true" SUB-woofer is another story. I'm sure that it will more than supplement the little bookshelf's that you Dad is currently using and should be capable of pretty solid bottom end in a box that size. If the driver does not hold up ( although i see no reason that it shouldn't other than foam rot ), you can always replace it with another driver that would be more suitable for this specific application. Sean
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