I have a blue book question

I’ve been tasked with selling my deceased step father’s vintage audio system for my mom. I’ve been able to find values for everything except the subwoofers. They are a pair of Duntec IB 20s with a crossover. I believe these were made around 1986.

 The entire set up includes a Quad 98 preamp, a pair of Quad 405 100 watt per channel amplifiers, each driving two of the stacked Quad ESL 57 electrostatic speakers and 2 Duntech IB 20 subwoofers with external crossover unit. Chase RLX-1 audio line switcher for up to six inputs. Custom built speaker and sub stands. My step dad built large pipe organs for a living. He made the stands himself. He loved this system so much. All the components work. The subwoofers were reconed last year. Any help pricing these subs and crossover would be greatly appreciated.