With your items, I agree on 5 out of 6. AP is important, but I'd change Lich's bane to another sunfire cape, and another would even consider changing abysal sceptre to banshee's veil.

The reasoning behind this is, Amumu is a support. His job is to get into the middle of a massive tangle at the centre of the two teams and trap all of them in his ulti. If you're in the middle, there's a chance they're going to stun you. AP is good, it makes despair and tantrum better of course but keeping the enemy there and staying alive and being able to stun when it recovers to finish the last one and even just being a presence is more useful than less effective damage output. And besides, you won't be able to use your AP if you don't survive! Your higher damaging team mates will do the damage, you just trap give them fish in a barrel. That's Amumu's job.

You could've Saved as Draft to keep for yourself if this build was for personal reference.