I have JansZen hybrid Valentina ESLs mated to twin Rythmik 12G subs. I think the key for my setup was a combination of settings. First, crossing over the subs just below the 30Hz level with the steepest filter slope, the bottom range of flat response for the integrated woofers on the JansZens.
Second, setting the bass extension controls with maximum damping allowed to subs to play in a similar manner as the speaker woofers. Gettng the volume control on the subs so that the contribution felt simply like extension and not additional bass was also key.
I ran the JanZens without subs for quite a few years before getting the subs but I wanted more impact and bass foundation. I was afraid they would make things too thick sounding. They don't when dialed in right.