Forget about the 740 if the vocals are important to you. The harmonizer is VERY weak. I think it's awful, but many would argue that point. Many would argue anything just for sport, but I'd give it a good test before you dish out the cash. I'm tellin' you, man - the 740 is NOT for singers.
** sounds like you should get a workstation type board and use the DJX patterns in tandem - this way you'll get new sounds and proccessing power, with some old "friends" in the DJX rhythm section.

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Uncle Dave