In an article by Bobby Owsinski about his favorite EQ he mentions that he uses a Massive Passive on vocals. He said "This is my go-to plugin for vocals. It can add sparkle and heft to a vocal done even on an SM58 in a way that few others can. It's perfect for carving out space in the mix for a track." Using the Massive Passive on vocals seems a little overkill to me, but Bobby knows what he's doing so I'll certainly have to give this a try.