I've only fished a few times for the Cats in this country, my takes occurred when fishing 3oz<or so> Rudd livebaits popped up near the surface. That said, the lake I fished was only a couple of feet deep and the Rudd predominate, no drama there then, the Cats must be used to slurping them from the top. Indeed, every now and again, the silence was shattered as a big Cat rolled on the surface, as it scranned another Rudd.
In Spain, the rig which has proved the best to me was a 'free roving' Eel, hooked with a single hook through the tail.