eventide vocal doubling

So I got a DSP 7000. I mostly use it for f**ked Fx and reverbs. I never have really done the "harmonisation" thing on the eventide but when I have experimented with doing harmonies and double of a vocal I never really like the sound. When the pros use the eventide for a double what are the technical setting like usually. I am more concerned with whats the theopry of a setting behind this simulated double. I mean I have been told that the Blink 182 sound is the eventide ont he vocals and I do love that sound for that type of band. Anyone know whats the deal.