Can I ask why an archtop, $600 is not a lot of money for that type of guitar? They're generally more expensive than decent flat tops even when you do find them. Cheap and loud is the ticket for busking ime.
If you're dead set on a jazz box, try the Peerless and Eastman ranges, they may have something in your budget. Epiphone do the Emperor guitars, I'm sure one model has a floating pickup that doesn't interfere with the acoustic tone.