First time I get to say WELCOME! I am new here myself. Please feel free to visit a thread I started and comment.

http://www.axiomaudio.com/boards/ubbthreads.php?ubb=showflat&Number=238133#Post238133

I'msure that you will get a lot of help from these great folks on this forum. The consensus seems to be that the M80's will put out plenty of bass and surely enough to keep you satisfied until you can solve your sub dilema. From my limited knowledge, which includes dabbling in PA Systems, I would think that even if you could manage to get both channels funnelled as one to your sub, that it wouldn't add a thing at 110 watts total to what you will be getting out of the M80's. Also subs have come a long way in 20 years. How many watts was the original amp in the sub and what is the speaker in the sub capable of handling?

My guess is that you would probably be best of saving any money that would be invested in that sub and put it towards a new sub.