Brewers at The Boston Beer Company ran trials to determine how common spices changed perceived bitterness in various beers. They found that there’s more to bitterness than isomerized alpha acids and more to measuring IBU than those isomerized compounds.
The Secret of Hops is Malt
Mark Hastings of Überbrew argues that the selection and combination of hops varieties isn't the end-all, be-all of brewing hoppy styles. The real magic is finding the right malt and yeast combination that pulls the best character out of those hops.