use only bass in 3 way speaker?

Thank you Duke and Xiekitchen. Regarding the crossover topology, I have no idea what it means so I better leave it alone. But thanks. Xiekitchen, I have no such controls on my old speakers, so as above I will leave it alone. I did not realize that it was so complicated. I will have to buy a electrical thingy that will only allow the bass speaker to work. Regards Peter

I dont like that idea.
Just sell the Isophons and get a real subwoofer.

I think you can drive your Isophons with a subwoofer amp, which would allow you to roll off everything above the subwoofer region. No modifications necessary, the variable low-pass filter in the amp, along with its other controls, would give you the tools needed to get a decent blend with your bookshelf speakers.

I've used bass guitar cabinets as subwoofers in a pinch, same basic idea.

Duke