Convert Bozak Mono Speaker to Subwoofer???

My dad has an old Bozak speaker, for which he built a 44" (w), 22" (d) and 17" (h) box. The box contains a 12" woofer, a 5" midrange, and 2 tweeters mounted over the woofer. He no longer uses this speaker, but instead has two bookshelf speakers (actually sitting on bookshelves!). That's not going to change, but I was thinking about using the woofer in this Bozak to get him a little more bass extension. The idea is to purchase a subwoofer amp from Part's Express, take the signal from the preamp out or the speaker terminals of his receiver, and do whatever I can to get the sound balanced. The Bozak would sit in the corner of a rectangular room (approx 14x28), and cannot be rearranged. If necessary, we could take out the midrange and plug up the hole, just leaving the woofer in the existing box.

What do you all think. Is this potentially worth it, or does the box shape, size, and inflexibility with location make it not worthwhile. If it's worthwhile, any additional advice would be welcome! Thanks, Peter.