All of my Itohs die back to the ground every winter, I've never had to prune them. You should be able to cut back pretty severely but I'd go by what your peony did last year. Did you get any new growth on existing branches or did all of the new growth come from the bottom couple of inches of stem and from eyes on the root? If so, you should be able to prune them down to just the bottom couple of inches.

Sorry I don't have any really relevant experience because of the different zones we're in. Mainly wanted to ask what zone you're in that they don't die back, what varieties you have and ask you to post pics if you have any!