You probably have this.
http://www.davidsaudio.com/IRS_Beta_Manual.pdf
So from what you describe and the age etc, to me sounds like a leaky cap or caps in the circuit. A good audio/ electrician repair tech should be able to test and find the bad part or parts.
I think I recall back in the day some folks pulled these apart and upgraded most of the caps etc with better grade parts as well. I am sure you have surfed around and I see non on eBay, the agon etc. it would be nice to stay near stock, maybe at least call Madisound maybe they know someone too if they won't offer repairs. I am sure there are folks that fix these, also what about audio classics, they not only sell used gear, they repair as well.
Good luck, it would be a major chore to build a passive crossover, it can be done, I have built many, but in subs it is most times these days cheaper to go active.
Best luck keep us posted
Sent from my iPad using A.Aficionado