Actually, folks, those weren't bad speakers back in the day. They were regarded as having one of the best SQ/price quotients out there, and I wouldn't turn down a free pair myself. The foam surrounds might need repair though, but there are kits readily available.
You'll want something pretty powerful going into them to make them pound. Perhaps something like
this
With 15" woofers and a 20 Hz frequency response, I doubt a subwoofer could make a great difference if you've got enough power to begin with.
EDIT: Perhaps I stand pre-corrected (I always get distracted while I compose replies, so I'm a little slow at times) on the power issue. I don't have any actual experience with these speakers, but I've heard all about them. Still, Harman/Kardon has great receivers, and I think the speakers are rated at 100 WPC continuous.