I too, don't think it's likely that your old sub will add anything. Subs themselves have come quite a long way in this time...not just the amps. Your old sub likely doesn't go lower than 35Hz or so... which is about where the 80's will take you anyway.
Unless your classical tastes take you heavily into organ music, you likely won't feel the need for a sub at all as far as frequency response goes. If you like your peaks to be quite loud, a sub will take much of the burden off your receiver, though in the last few octaves.
How big is your room?
