Subwoofer integration with Silverline Sonata MK111

Can anyone who has successfully integrated a subwoofer with the Silverline Audio Sonta Mk111 kindly share their experience?

I am trying to integrate a REL Storm 3 sub and would like to know what the optimum cross over between the sub and Sonata should be.

The Sonata specs are supposedly from 25 to 20,000 Hz.

Thanks