Have you tried singing to a Bb? Or has the person who sings for you? I'd suggest you try it out first. I have a pretty broad range and I can't sing anything when played on a Bb. It's always too high or too low. May I suggest you consider an E? Just a thought.
I have the same problem with Bb.. which for lots of guys is the perfect solution for singing. I too have a broad range..but can handle E, as Sarah suggests..