Lots of health is important for Volibear, and Spirit Visage + Warmog's is pretty much core on him, as they both help his damage and survivability immensely. Tanky hybrid, in my opinion, is stronger than either AP or AD on Voli:
The whole point of Voli is you build him tanky and his W will just sort of do massive damage anyway. The rest of his kit is about catching people and hitting them a bunch of times while you wait to use your W. Some AD and attack speed is good too, but your autos and ult is more about keeping some steady damage while waiting for that bite.
AP scaling is pretty much worthless to him since it can't be used consistently against people. Your ult is only good when there are a couple people near by who have fully committed to a fight and it doesn't even last a long time. And your E is never going to compare to your W even taking cooldowns and the AA build up into consideration.