I've come to learn that the Yamaha mic input is stingy. It only "likes" even, moderate signals and if you shout, or push a vocal .... it overloads. I had to revise my style of singing ( which really BUGS me )during harmony usage. It's an inferior system to the standalones, but it's not a malfunction. Just sing ez and steady and don't push it or it'll sound like crap. If your vocal styling is too dynamic, like mine can get .... you need to either calm down or use something else.
It's not broken....just mediocre. I opt to use it in a reduced fashion in stead of lugging more gear, but my the studio - it's never used. I have a TC Helicon and a Digitech that work great.