Say what you will about Gerard Butler, he at least can hold a note properly. Russell Crowe just yells aggressively and thinks it's music

"he at least can hold a note properly." Lol, wut? When? When does he ever do this?
If anything, Russell is the one that sounds Grammy and Tony worthy in comparison to Gerard Butler.

Anyways, both would be completely unpleasant to listen to, but at least Emmy is subpar when it comes to singing, so she'd probably make Amanda's weak and terrible voice sound a little less awful (though probably not by much).
Emmy's odd diction and breathy singing and Amanda's weak vocals and awful vibrato have nothing on Crowe and Butler, though. Russell and Gerard's guttural yelling grinding against each other would sound absolutely terrifying while causing eardrum genocide.

Russell and Amanda are AMAZING when it comes to acting (Gerard isn't too shabby of an actor himself; he just needs to learn to stop overacting), but singing... no. Just, no.

I do find it a little ironic how the only one who can't act (Rossum) is the least awful singer of the bunch.

Neither.