Scott,

Why can't you just position the JBLs on either side of you and a little to the rear, then face them at slight angles toward the rear/center of the room where you're playing? Then you would be able to hear your keyboard and vocals alike without using monitors at all. I've done this a few times when I was using the 740s and had no problems with feedback--but of course this will depend on how you EQ the vocals.

Gary