A word of warning – I built up an Enduro some years back, fitted a Pig DH – bottom cup was *really* tight – couldn't get it quite flat against the headtube, or even remove it with the proper rocket tool.
LBS monkey couldn't shift it (monkey on account of taking lots of little nicks out of the inside of the headtube with whatever screwdriver the idiot had been using).
Spesh UK kindly whipped it out (half handlebar as a drift works rather better it seems) and refaced it just in case. It looked like the powerdercoat on the cup had kinda galled on the inside of the headtube – pretty sure they come in a little oversize compared to some headsets as a result of the powdercoat.
So before you start lobbing a hammer at it, bear in mind whether there's any reason it's stuck…