use only bass in 3 way speaker?

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

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

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

well I did something like that for fun about a decade ago, I just rotated the two alps pots in my JBL L-166 Horizons to zero so that just the woofer was playing.. faced it backwards.. and plopped my Spica's on top facing forwards.. it was an experiment shall we say. Sounded OK.

If the filter topology to the midrange and/or tweeter in your old speakers is second order or higher, and if there are no damping resistors on the parallel legs of the filter, there will be a frequency or frequencies at which the load presented to the amplifier is virtually a dead short.

Also in general, you'd want an optimized blend between woofer and midrange, which means a crossover designed for the task with those specific drivers in mind, and combining the woofer section of an existing speaker with a new bookshelf speaker won't give you that.

Duke