As far as I know, Anthony is the only member of the cast with an extensive stage-musical background. (Someone correct me if I'm wrong - I know that James and Amber are also really good singers.)
One thing that bugs me about OMWF is that Joss TOTALLY under-used Anthony's stunning singing voice. If you've ever heard him in Repo, or in the Chess 1989 bootleg, his voice is amazing - rich and powerful, and he must have three octaves in his range.
So, IMO Anthony is by far the best singer in this show, but underused.
I agree that Under Your Spell would have been so much better in a lower key.