EDIT: Nevermind i just saw your asking for a single channel... my bad.
How about a Focusrite ISA One? You could eventually upgrade it with the digital card and have some good converters built into it. They'd work great on vocals. Its about the only thing i can think of without going lunchbox that's in that price range that would be a considerable upgrade to what you already have. Not much under 500$ will make much of a difference compared to your saffire pre's.