The problem with those seats is that some tools tend to bottom out at the back of the valve, before fully engaging. If your valve is a typical like PP, and you used an un-modified 3-step wrench or tapered wrench, that is how you damaged the hole.
At this point, a #6 Easy-out may work. I think some of the other guys may have other hints. Not that installing a new pressure balanced valve is a bad idea...but you can save a lot of money for now if you can get the seats out.