You nerfed the wrong part of Amumu. (We don't even use W till late game Riot)
I'm kinda confused on why you stated you wanted to nerf his clear speeds but then nerf the move that doesn't affect his clear speeds. I'm not sure if you guys are aware but Amumu's clear speed is due to his E not his W.(Yes, most people max E that have experience with Amumu)
Just curious if you're aware of this and if you still have Amumu issues then make sure to nerf the right thing next time.